theme_experiment

Type	`Object`
Mandatory	No
Manifest version	2 or higher
Example	json `"theme_experiment": { "stylesheet": "style.css", "colors": { "popup_affordance": "--arrowpanel-dimmed" }, "images": { "theme_toolbar": "--toolbar-bgimage" }, "properties": { "toolbar_image_alignment": "--toolbar-bgalignment" } }`

This key enables the definition of experimental theme key properties for the Firefox interface. These experiments are a precursor to proposing new theme features for inclusion in Firefox. Experimentation is done by:

creating a stylesheet that defines mappings between internal CSS selectors for Firefox UI elements and arbitrary CSS variables. The CSS variables are then mapped in the colors, images, and properties objects to new theme key properties.
(without a stylesheet) using colors, images, and properties to map internal Firefox CSS selectors, such as --arrowpanel-dimmed to new theme key properties. This option limits experimentation to UI components that are associated with an inbuilt CSS variable.

To discover the CSS selectors for Firefox UI elements or internal Firefox CSS variables use the browser toolbox.

Note: This key is only available for use in Firefox Developer Edition and Firefox Nightly channels and requires the extensions.experiments.enabled preference to be enabled. In Firefox 73 and earlier, the extensions.legacy.enabled had to be used instead.

Warning: This feature is experimental and could be subject to change.

Syntax

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

Name	Type	Description
`stylesheet`	`String`	Optional Name of a stylesheet providing mapping of Firefox UI element CSS selectors to CSS variables.
`images`	`Object`	Optional Mappings of CSS variables (as defined in Firefox or by the stylesheet defined in `stylesheet`) to `images` property names for use in the `theme` key.
`colors`	`Object`	Optional Mappings of CSS variables (as defined in Firefox or by the stylesheet defined in `stylesheet`) to `colors` property names for use in the `theme` key.
`properties`	`Object`	Optional Mappings of CSS variables (as defined in Firefox or by the stylesheet defined in `stylesheet`) to `properties` property names for use in the `theme` key.

Examples

This example uses a stylesheet named style.css to provide the ability to set a color for the browser reload button in the theme key.

The stylesheet defines:

css

#reload-button {
  fill: var(--reload-button-color);
}

where #reload-button is the Firefox internal CSS selector for the reload button and --reload-button-color is an arbitrary name.

In the manifest.json file, --reload-button-color is then mapped to the name to be used in the colors property of theme:

json

"theme_experiment": {
  "stylesheet": "style.css",
  "colors": {
    "reload_button": "--reload-button-color"
  }
}

The argument reload_button is used in the same way as any other theme property:

json

"theme": {
  "colors": {
    "reload_button": "orange"
  }
}

This has the effect of making the reload icon orange.

Outcome of a theme experiment, showing the reload button colored orange.

This property can also be used in browser.theme.update(). images and properties work in a similar way to colors.

Browser compatibility

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This page was last modified on ⁨Jul 17, 2025⁩ by MDN contributors.

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