Injects CSS into a page.
To use this API you must have the permission for the page's URL, either explicitly as a host permission, or using the activeTab permission.
You can only inject CSS into pages whose URL can be expressed using a match pattern: meaning, its scheme must be one of "http", "https", or "file". This means that you can't inject CSS into any of the browser's built-in pages, such as about:debugging, about:addons, or the page that opens when you open a new empty tab.
Note: Firefox resolves URLs in injected CSS files relative to the CSS file itself, rather than to the page it's injected into.
The inserted CSS may be removed again by calling tabs.removeCSS()
.
This is an asynchronous function that returns a Promise
(on Firefox only).