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theme

Type Object
Mandatory No
Manifest version 2 or higher
Example 
"theme": {
  "images": {
    "theme_frame": "images/sun.jpg"
  },
  "colors": {
    "frame": "#CF723F",
    "tab_background_text": "#000"
  }
}

Use the theme key to define a static theme to apply to Firefox.

Note: If you want to include a theme with an extension, please see the theme API.

Note: Since May 2019, themes need to be signed to be installed (Firefox bug 1545109). See Signing and distributing your add-on for more details.

Note: A new version of Firefox for Android, based on GeckoView, is under development. A pre-release version is available. The pre-release version does not support themes.

Image formats

The following image formats are supported in all theme image properties:

  • JPEG
  • PNG
  • APNG
  • SVG (animated SVG is supported from Firefox 59)
  • GIF (animated GIF isn't supported)

Syntax

The theme key is an object that takes the following properties:

Name Type Description
images Object

Optional as of Firefox 60. Mandatory before Firefox 60.

A JSON object whose properties represent the images to display in various parts of the browser. See images for details on the properties that this object can contain.
colors Object

Mandatory.

A JSON object whose properties represent the colors of various parts of the browser. See colors for details on the properties that this object can contain.
properties Object

Optional

This object has properties that affect how the "additional_backgrounds" images are displayed and color schemes are applied. See properties for details on the properties that this object can contain.

images

All URLs are relative to the manifest.json file and cannot reference an external URL.

Images should be 200 pixels high to ensure they always fill the header space vertically.

Name Type Description
headerURL String

Warning: headerURL has been removed in Firefox 70. You will begin to get warnings in Firefox 65 and later if you load a theme that uses this property. Use theme_frame instead.

The URL of a foreground image to be added to the header area and anchored to the upper right corner of the header area.

Optional in desktop Firefox from Firefox 60 onwards. One of theme_frame or headerURL had to be specified before Firefox 60. Note also that in Firefox 60 onwards, any text-shadow applied to the header text is removed if no headerURL is specified (see [Firefox bug 1404688](https://bugzil.la/1404688)).

In Firefox for Android, headerURL or theme_frame must be specified.
theme_frame String

The URL of a foreground image to be added to the header area and anchored to the upper right corner of the header area.

Note: Chrome anchors the image to the top left of the header and if the image doesn't fill the header area tile the image.

Optional in desktop Firefox 60 onwards. One of theme_frame or headerURL had to be specified before Firefox 60.

In Firefox for Android, headerURL or theme_frame must be specified.
additional_backgrounds Array of String

Warning: The additional_backgrounds property is experimental. It is currently accepted in release versions of Firefox, but its behavior is subject to change. It is not supported in Firefox for Android.

An array of URLs for additional background images to be added to the header area and displayed behind the "theme_frame": image. These images layer the first image in the array on top, the last image in the array at the bottom.

Optional.

By default all images are anchored to the upper right corner of the header area, but their alignment and repeat behavior can be controlled by properties of "properties":.

colors

These properties define the colors used for different parts of the browser. They are all optional (but note that "accentcolor" and "textcolor" were mandatory in Firefox before version 63). How these properties affect the Firefox UI is shown here:

Overview of the color properties and how they apply to Firefox UI components

Note: Where a component is affected by multiple color properties, the properties are listed in order of precedence.

All these properties can be specified as either a string containing any valid CSS color string (including hexadecimal), or an RGB array, such as "tab_background_text": [ 107 , 99 , 23 ].

Note: In Chrome, colors may only be specified as RGB arrays.

In Firefox for Android colors can be specified using:

  • full hexadecimal notation, that is #RRGGBB only. alpha and shortened syntax, as in #RGB[A], are not supported.
  • Functional notation (RGB arrays) for themes targeting Firefox 68.2 or later.

Colors for Firefox for Android themes cannot be specified using color names.

Name Description

accentcolor

Warning: accentcolor has been removed in Firefox 70. You will begin to get warnings in Firefox 65 and later if you load a theme that uses this property. Use the frame property instead.

The color of the header area background, displayed in the part of the header not covered or visible through the images specified in "headerURL" and "additional_backgrounds".

See example 
"theme": {
  "colors": {
     "accentcolor": "red",
     "tab_background_text": "white"
  }
}

Browser firefox is red with white text. Browsers tabs are lighter red, also with white text. URL bar is very light red with black text
bookmark_text

The color of text and icons in the bookmark and find bars. Also, if tab_text isn't defined it sets the color of the active tab text and if icons isn't defined the color of the toolbar icons. Provided as Chrome compatible alias for toolbar_text.

Note: Ensure any color used contrasts well with those used in frame and frame_inactive or toolbar if you're using that property.

Where icons isn't defined, also ensure good contrast with button_background_active and button_background_hover.

See example 
"theme": {
  "colors": {
    "frame": "black",
    "tab_background_text": "white",
    "tab_text": "white",
    "toolbar": "black",
    "bookmark_text": "red"
  }
}

Browser Firefox is black. Browser's tab is black with white text. URL bar and the find in page bar are white with black text but all the browser and the find in page bar icons are red.
button_background_active

The color of the background of the pressed toolbar buttons.

See example 
"theme": {
  "colors": {
     "frame": "black",
     "tab_background_text": "white",
     "button_background_active": "red"
  }
}

Browser firefox is black. Browser's tabs and URL bar are grey with white text. The customize toolbar icon in the url bar in white with a red background is pressed and a popup is open displaying a short list of thing to add to the toolbar such as the browser's library and the sidebars.
button_background_hover

The color of the background of the toolbar buttons on hover.

See example 
"theme": {
  "colors": {
     "frame": "black",
     "tab_background_text": "white",
     "button_background_hover": "red"
  }
}

Browser firefox is black. Browser's tabs and URL bar are grey with white text. The go back one page icon is white with a red circle background.
icons

The color of toolbar icons, excluding those in the find toolbar.

Note: Ensure the color used contrasts well with those used in frame, frame_inactive, button_background_active, and button_background_hover.

See example 
"theme": {
  "colors": {
     "frame": "black",
     "tab_background_text": "white",
     "icons": "red"
  }
}

Browser firefox is black. Browser's tabs and URL bar are grey with white text. The URL bar and open a new tab icons are red. The red icons contrast well with the black background color of the header area.
icons_attention

The color of toolbar icons in attention state such as the starred bookmark icon or finished download icon.

Note: Ensure the color used contrasts well with those used in frame, frame_inactive, button_background_active, and button_background_hover.

See example 
"theme": {
  "colors": {
     "frame": "black",
     "tab_background_text": "white",
     "icons_attention": "red"
  }
}

Browser firefox is black. Browser's tabs and URL bar are grey with white text. The bookmark this page icon is red and pressed, an open popup name edit this bookmark is displayed. While in attention state, the toolbar icons contrast well with the black background of the header area.
frame

The color of the header area background, displayed in the part of the header not covered or visible through the images specified in "theme_frame" and "additional_backgrounds".

See example 
"theme": {
  "colors": {
     "frame": "red",
     "tab_background_text": "white"
  }
}

Browser firefox is red with white text. Browsers tabs are lighter red, also with white text. URL bar is very light red with black text
frame_inactive

The color of the header area background when the browser window is inactive, displayed in the part of the header not covered or visible through the images specified in "theme_frame" and "additional_backgrounds".

See example 
"theme": {
  "colors": {
     "frame": "red",
     "frame_inactive": "gray",
     "tab_text": "white"
  }
}

Browser firefox is grey. Browser's tabs and URL bar are lighter grey. The tab text is white and the URL bar icon are darker grey.
ntp_background

The new tab page background color.

See example 
"theme": {
  "colors": {
     "ntp_background": "red",
     "ntp_text": "white"
  }
}

Browser firefox is white. The new tab page background is red, with google search at the top, follow by top sites shortcut and recommended articles.
ntp_text

The new tab page text color.

Note: Ensure the color used contrasts well with that used in ntp_background.

See example 
"theme": {
  "colors": {
     "ntp_background": "red",
     "ntp_text": "white"
  }
}

Browser firefox is white. The new tab page background is red, with google search at the top, follow by top sites shortcut and recommended articles. The color of the text in the new tab page is white.
popup

The background color of popups (such as the URL bar dropdown and the arrow panels).

See example 
"theme": {
  "colors": {
     "frame": "black",
     "tab_background_text": "white",
     "popup": "red"
  }
}

Browser firefox is black. Browser's tabs and URL bar are lighter grey with icons and text in white. The bookmark this page icon is blue and pressed, an open popup name 'edit this bookmark' is displayed with a red background. The background color of the popup is red.
popup_border

The border color of popups.

See example 
"theme": {
  "colors": {
     "frame": "black",
     "tab_background_text": "white",
     "popup": "black",
     "popup_text": "white",
     "popup_border": "red"
  }
}

Browser firefox is black. Browser's tabs and URL bar are lighter grey with icons and text in white. The bookmark this page icon is blue and pressed, an open popup name 'edit this bookmark' is displayed with a red outline and black background. The popup's border is red.
popup_highlight

The background color of items highlighted using the keyboard inside popups (such as the selected URL bar dropdown item).

Note: It's recommended to define popup_highlight_text to override the browser default text color on various platforms.

See example 
"theme": {
  "colors": {
     "frame": "black",
     "tab_background_text": "white",
     "popup_highlight": "red",
     "popup_highlight_text": "white"
  }
}

screenshot of firefox is black. Browser's tabs and URL bar are lighter grey with icons and text in white. A search results popup is displayed with a highlighted item's background in red. The background color of the highlighted item inside the popup is red.
popup_highlight_text

The text color of items highlighted inside popups.

Note: Ensure the color used contrasts well with that used in popup_highlight.

See example 
"theme": {
  "colors": {
     "frame": "black",
     "tab_background_text": "white",
     "popup_highlight": "black",
     "popup_highlight_text": "red"
  }
}

Browser firefox is black. Browser's tabs and URL bar are lighter grey with icons and text in white. A search results popup is displayed with a highlighted item's text in red with a black background. The text color of the highlighted item contrasts well with the black background color of this item.
popup_text

The text color of popups.

Note: Ensure the color used contrasts well with that used in popup.

See example 
"theme": {
  "colors": {
     "frame": "black",
     "tab_background_text": "white",
     "popup": "black",
     "popup_text": "red"
  }
}

Browser firefox is black. Browser's tabs and URL bar are lighter grey with icons and text in white. A search results popup is displayed with items texts in red. The text color contrasts well with the black background color of the popup.
sidebar

The background color of the sidebar.

See example 
"theme": {
  "colors": {
     "sidebar": "red",
     "sidebar_highlight": "white",
     "sidebar_highlight_text": "green",
     "sidebar_text": "white"
  }
}

A close-up screenshot of a browser windows's open sidebar. The background color of the sidebar is red.
sidebar_border

The border and splitter color of the browser sidebar

See example 
"theme": {
  "colors": {
     "sidebar_border": "red"
  }
}

A closeup of the firefox browser bookmarks sidebar with a red horizontal separator between the sidebar title and the sidebar menu. The border and splitter color of the sidebar is red.
sidebar_highlight

The background color of highlighted rows in built-in sidebars

See example 
"theme": {
  "colors": {
     "sidebar_highlight": "red",
     "sidebar_highlight_text": "white"
  }
}

A closeup of the firefox browser bookmarks sidebar with a highlighted item. The background color of a highlighted row in the sidebar is red with white text.
sidebar_highlight_text

The text color of highlighted rows in sidebars.

Note: Ensure the color used contrasts well with that used in sidebar_highlight.

See example 
"theme": {
  "colors": {
    "sidebar_highlight": "pink",
    "sidebar_highlight_text": "red",
  }
}

A closeup of the firefox browser bookmarks sidebar with a highlighted item. The color of the text of a highlighted row in the sidebar is red. The text color contrasts well with the pink background color of the highlighted row.
sidebar_text

The text color of sidebars.

Note: Ensure the color used contrasts well with that used in sidebar.

See example 
"theme": {
  "colors": {
     "sidebar": "red",
     "sidebar_highlight": "white",
     "sidebar_highlight_text": "green",
     "sidebar_text": "white"
  }
}

A close-up screenshot of a browser windows's open sidebar. The color of the text inside the sidebar is white. The text color contrasts well with the red background of the sidebar.
tab_background_separator

Warning: tab_background_separator is not supported starting with Firefox 89.

The color of the vertical separator of the background tabs.

See example 
"theme": {
  "colors": {
     "frame": "black",
     "tab_background_text": "white",
     "tab_background_separator": "red"
  }
}

A closeup of browser tabs to highlight the separator.
tab_background_text

The color of the text displayed in the inactive page tabs. If tab_text or bookmark_text isn't specified, applies to the active tab text.

Note: Ensure the color used contrasts well with those used in tab_selected or frame and frame_inactive.

See example 
"theme": {
  "colors": {
    "frame": "black",
    "toolbar": "white",
    "tab_background_text": "red"
  }
}

A screenshot of a browser window with one open tab. Browser is black. Browser's tabs and URL bar are white with red icons and red text. The color of the text in the open tab is red. The text color contrasts well with the black background color of the tab.
tab_line

The color of the selected tab line.

See example 
"theme": {
  "colors": {
     "frame": "black",
     "tab_background_text": "white",
     "tab_line": "red"
  }
}

Browser firefox is black. Browser's tabs and URL bar are darker grey with lighter grey icons and white text. The selected tab has a red outline.
tab_loading

The color of the tab loading indicator and the tab loading burst.

See example 
"theme": {
  "colors": {
     "frame": "black",
     "tab_background_text": "white",
     "tab_loading": "red"
  }
}

A screenshot of a browser window with one open tab. Browser is black. Browser's tabs and URL bar are darker grey with icons and text in white. Inside the selected tab an animated loading indicator is red.
tab_selected

The background color of the selected tab. When not in use selected tab color is set by frame and the frame_inactive.

See example 
"theme": {
  "images": {
  "theme_frame": "weta.png"
},
  "colors": {
     "frame": "black",
     "tab_background_text": "white",
     "tab_selected": "red"
  }
}

A screenshot of a browser window with one open tab. Browser is black. Browser's tabs and URL bar are darker grey with icons and text in white. The selected tab has red background and white text.
tab_text

From Firefox 59, it represents the text color for the selected tab. If tab_line isn't specified, it also defines the color of the selected tab line.

Note: Ensure the color used contrasts well with those used in tab_selected or frame and frame_inactive.

From Firefox 55 to 58, it is incorrectly implemented as alias for "textcolor"

See example 
"theme": {
  "images": {
  "theme_frame": "weta.png"
},
  "colors": {
     "frame": "black",
     "tab_background_text": "white",
     "tab_selected": "white",
     "tab_text": "red"
  }
}

Browser firefox has a picture of an insect theme. URL bar is lighter grey with white icons. The selected tab text is red with white background.
textcolor

Warning: textcolor has been removed in Firefox 70. You will begin to get warnings in Firefox 65 and later if you load a theme that uses this property. Use tab_background_text instead.

The color of the text displayed in the header area.

See example 
"theme": {
  "colors": {
    "frame": "black",
    "toolbar": "white",
    "textcolor": "red"
  }
}

Browser firefox is black. Browser's tab and URL bar are white with red text and red icons.
toolbar

The background color for the navigation bar, the bookmarks bar, and the selected tab.

This also sets the background color of the "Find" bar.

See example 
"theme": {
  "colors": {
    "frame": "black",
    "toolbar": "red",
    "tab_background_text": "white"
  }
}

Browser firefox is black. Browser's tab, find in page bar and URL bar are red with white text and icons, except for the find in page bar where the text and icon are black.
toolbar_bottom_separator

The color of the line separating the bottom of the toolbar from the region below.

See example 
"theme": {
  "colors": {
    "frame": "black",
    "tab_background_text": "white",
    "toolbar_bottom_separator": "red"
  }
}

Browser firefox is black. Browser's tab and URL bar are lighter grey with white text and icons. A horizontal red line separates the bottom of the toolbar and the beginning of the display of the web page.
toolbar_field

The background color for fields in the toolbar, such as the URL bar.

This also sets the background color of the Find in page field.

See example 
"theme": {
  "colors": {
    "frame": "black",
    "tab_background_text": "white",
    "toolbar_field": "red"
  }
}

Browser firefox is black. Browser's tab, find in page bar and URL bar are lighter grey with white text and icons. The background color of the URL bar is red. The find in page bar is white with black text. The find in page field is red with black text.
toolbar_field_border

The border color for fields in the toolbar.

This also sets the border color of the Find in page field.

See example 
"theme": {
  "colors": {
    "frame": "black",
    "toolbar": "black",
    "tab_background_text": "white",
    "toolbar_field": "black",
    "toolbar_field_text": "white",
    "toolbar_field_border": "red"
  }
}

Browser firefox is black. Browser's tab, find in page and URL bar are black with white text and icons. The URL bar and find in page fields are outlined in red.
toolbar_field_border_focus

The focused border color for fields in the toolbar.

See example 
"theme": {
  "colors": {
    "frame": "black",
    "toolbar": "black",
    "tab_background_text": "white",
    "toolbar_field": "black",
    "toolbar_field_text": "white",
    "toolbar_field_border_focus": "red"
  }
}

Browser firefox is black. Browser's tab and URL bar are black with white text and icons. The url bar field is focused and outlined in red.
toolbar_field_focus

The focused background color for fields in the toolbar, such as the URL bar.

See example 
"theme": {
  "colors": {
    "frame": "black",
    "toolbar": "black",
    "tab_background_text": "white",
    "toolbar_field": "black",
    "toolbar_field_text": "white",
    "toolbar_field_focus": "red"
  }
}

Browser firefox is black. Browser's tab, find in page and URL bar are black with white text and icons. The background color of the focused URL bar is red and the text is white.
toolbar_field_highlight The background color used to indicate the current selection of text in the URL bar (and the search bar, if it's configured to be separate).
See example 
"theme": {
  "colors": {
    "toolbar_field": "rgba(255, 255, 255, 0.91)",
    "toolbar_field_text": "rgb(0, 100, 0)",
    "toolbar_field_highlight": "rgb(180, 240, 180, 0.9)",
    "toolbar_field_highlight_text": "rgb(0, 80, 0)"
  }
}

Browser firefox is white. Browser's tab and URL bar are white with text and icons in black. The URL bar field is focused and outlined in blue and URL bar text is selected.

Here, the toolbar_field_highlight field specifies that the highlight color is a light green, while the text is set to a dark-to-medium green using toolbar_field_highlight_text.
toolbar_field_highlight_text

The color used to draw text that's currently selected in the URL bar (and the search bar, if it's configured to be separate box).

Note: Ensure the color used contrasts well with those used in toolbar_field_highlight.

See example 
"theme": {
  "colors": {
    "toolbar_field": "rgba(255, 255, 255, 0.91)",
    "toolbar_field_text": "rgb(0, 100, 0)",
    "toolbar_field_highlight": "rgb(180, 240, 180, 0.9)",
    "toolbar_field_highlight_text": "rgb(0, 80, 0)"
  }
}

Browser firefox is white. Browser's tab and URL bar are white with text and icons in black. The URL bar field is focused and outlined in blue and URL bar text is selected.

Here, the toolbar_field_highlight_text field is used to set the text color to a dark medium-dark green, while the highlight color is a light green.
toolbar_field_separator

Warning: toolbar_field_separator is not supported starting with Firefox 89.

The color of separators inside the URL bar. In Firefox 58 this was implemented as toolbar_vertical_separator.

See example 
"theme": {
  "colors": {
    "frame": "black",
    "toolbar": "black",
    "tab_background_text": "white",
    "toolbar_field_separator": "red"
  }
}

A screenshot of a browser window with one open tab. Browser firefox is black. Browser's tab and URL bar are black with text and icons in white. Inside the white URL bar field, after the reader mode icon a red vertical line separating the rest of URL bar icons. The color of the vertical separator line inside the URL bar is red.

In this screenshot, "toolbar_vertical_separator" is the red vertical line in the URL bar dividing the Reader Mode icon from the other icons.
toolbar_field_text

The color of text in fields in the toolbar, such as the URL bar. This also sets the color of text in the Find in page field.

Note: Ensure the color used contrasts well with those used in toolbar_field.

See example 
"theme": {
  "colors": {
    "frame": "black",
    "toolbar": "black",
    "tab_background_text": "white",
    "toolbar_field": "black",
    "toolbar_field_text": "red"
  }
}

A screenshot of a browser window with one open tab. Browser is black. Browser's tab and URL bar are black with white text and icons. The text inside the URL bar is red. The icons and find in page field have red text with black background.
toolbar_field_text_focus

The color of text in focused fields in the toolbar, such as the URL bar.

Note: Ensure the color used contrasts well with those used in toolbar_field_focus.

See example 
"theme": {
  "colors": {
    "frame": "black",
    "toolbar": "black",
    "tab_background_text": "white",
    "toolbar_field": "black",
    "toolbar_field_text": "white",
    "toolbar_field_text_focus": "red"
  }
}

A screenshot of a browser window with two open tabs. Browser is black. Browser's tab and URL bar are black with text and icons in white. The URL bar has focus; the bar's text and icons are red with black background.
toolbar_text

The color of toolbar text. This also sets the color of text in the "Find" bar.

Note: For compatibility with Chrome, use the alias bookmark_text.

See example 
"theme": {
  "colors": {
    "frame": "black",
    "tab_background_text": "white",
    "toolbar": "black",
    "toolbar_text": "red"
  }
}

A screenshot of a browser window with one open tab. Browser is black. Browser's tab, find in page bar, and URL bar are black with red text and icons. The text inside the active tab, the navigator bar and the find bar is red.
toolbar_top_separator

The color of the line separating the top of the toolbar from the region above.

See example 
"theme": {
  "colors": {
    "frame": "black",
    "tab_background_text": "white",
    "toolbar": "black",
    "toolbar_top_separator": "red"
  }
}

A screenshot of a browser window with one open tab. Browser is black. Browser's tab and URL bar are black with white text and icons. A red line separates the top of the URL bar from the browser.
toolbar_vertical_separator

The color of the separator in the bookmarks toolbar. In Firefox 58, it corresponds to the color of separators inside the URL bar.

See example 
"theme": {
  "colors": {
    "frame": "black",
    "tab_background_text": "white",
    "toolbar": "black",
    "toolbar_vertical_separator": "red"
  }
}

A screenshot of a browser window with one open tab. Browser is black. Browser's tab and URL bar are black with text and icons in white. The color of the vertical line separating the bookmarks toolbar from the content to the right is red.

Aliases

Additionally, this key accepts various properties that are aliases for one of the properties above. These are provided for compatibility with Chrome. If an alias is given, and the non-alias version is also given, then the value will be taken from the non-alias version.

Warning: Beginning Firefox 70, the following properties are removed: accentcolor and textcolor. Use frame and tab_background_text instead. Using these values in themes loaded into Firefox 65 or later will raise warnings.

Name Alias for
bookmark_text toolbar_text
frame accentcolor
frame_inactive accentcolor
tab_background_text textcolor

properties

Name Type Description
additional_backgrounds_alignment

Array of String

Optional

An array of enumeration values defining the alignment of the corresponding "additional_backgrounds": array item.
The alignment options include:

  • "bottom"
  • "center"
  • "left"
  • "right"
  • "top"
  • "center bottom"
  • "center center"
  • "center top"
  • "left bottom"
  • "left center"
  • "left top"
  • "right bottom"
  • "right center"
  • "right top".

If not specified, defaults to "right top".
additional_backgrounds_tiling

Array of String

Optional

An array of enumeration values defining how the corresponding "additional_backgrounds": array item repeats. Options include:

  • "no-repeat"
  • "repeat"
  • "repeat-x"
  • "repeat-y"

If not specified, defaults to "no-repeat".
color_scheme

String

Optional

Determines which color scheme is applied to the chrome (for example, context menus) and content (for example, built-in pages and the preferred color scheme for web pages). Options include:

  • "auto" – a light or dark scheme based automatically on the theme.
  • "light" – a light scheme.
  • "dark" – a dark scheme.
  • "system" – uses the system scheme.

If not specified, defaults to "auto".
content_color_scheme

String

Optional

Determines which color scheme is applied to the content (for example, built-in pages and preferred color scheme for web pages). Overrides color_scheme. Options include:

  • "auto" – a light or dark scheme based automatically on the theme.
  • "light" – a light scheme.
  • "dark" – a dark scheme.
  • "system" – the system scheme.

If not specified, defaults to "auto".

Examples

A basic theme must define an image to add to the header, the accent color to use in the header, and the color of text used in the header:

 "theme": {
   "images": {
     "theme_frame": "images/sun.jpg"
   },
   "colors": {
     "frame": "#CF723F",
     "tab_background_text": "#000"
   }
 }

Multiple images can be used to fill the header. Before Firefox version 60, use a blank or transparent header image to gain control over the placement of each additional image:

 "theme": {
   "images": {
     "additional_backgrounds": [ "images/left.png", "images/middle.png", "images/right.png"]
   },
   "properties": {
     "additional_backgrounds_alignment": [ "left top", "top", "right top"]
   },
   "colors": {
     "frame": "blue",
     "tab_background_text": "#ffffff"
   }
 }

You can also fill the header with a repeated image, or images, in this case a single image anchored in the middle top of the header and repeated across the rest of the header:

 "theme": {
   "images": {
     "additional_backgrounds": [ "images/logo.png"]
   },
   "properties": {
     "additional_backgrounds_alignment": [ "top" ],
     "additional_backgrounds_tiling": [ "repeat"  ]
   },
   "colors": {
     "frame": "green",
     "tab_background_text": "#000"
   }
 }

The following example uses most of the different values for theme.colors:

  "theme": {
    "images": {
      "theme_frame": "weta.png"
    },

    "colors": {
       "frame": "darkgreen",
       "tab_background_text": "white",
       "toolbar": "blue",
       "bookmark_text": "cyan",
       "toolbar_field": "orange",
       "toolbar_field_border": "white",
       "toolbar_field_text": "green",
       "toolbar_top_separator": "red",
       "toolbar_bottom_separator": "white",
       "toolbar_vertical_separator": "white"
    }
  }

It will give you a browser that looks like this:

A browser window with two open tabs and dark green background color in the header area. The inactive tab has a white text color. The active tab and the toolbar have a blue background color with cyan-colored text. The URL bar has an orange background with white borders, a green text color and a white-colored vertical line separator. A red-colored line is used to separate the tabs on the top and a white line to separate the tabs from the content bellow them.

In this screenshot, "toolbar_vertical_separator" is the white vertical line in the URL bar dividing the Reader Mode icon from the other icons.

Browser compatibility

Desktop Mobile
Chrome Edge Firefox Internet Explorer Opera Safari WebView Android Chrome Android Firefox for Android Opera Android Safari on IOS Samsung Internet
theme Yes 79 55 ? No No ? ? Yes ? No ?
colors Yes 79 55 ? No No ? ? 65 ? No ?
images Yes 79
55Before Firefox 60, this property was required.
? No No ? ? Yes ? No ?
properties Yes 79 55 ? No No ? ? No ? No ?

Chrome compatibility

In Chrome:

  • colors/toolbar_text is not used, use colors/bookmark_text instead.
  • images/theme_frame anchors the image to the top left of the header and if the image doesn't fill the header area tile the image.
  • all colors must be specified as an array of RGB values, like this: 
    "theme": {
  "colors": {
     "frame": [255, 0, 0],
     "tab_background_text": [0, 255, 0],
     "bookmark_text": [0, 0, 255]
  }
}
    From Firefox 59 onward, both the array form and the CSS color form are accepted for all properties. Before that, colors/frame and colors/tab_background_text required the array form, while other properties required the CSS color form.

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https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/theme