Finally, there is a completely new feature for themes called theme_experiment that allows theme authors to declare their own theme properties based on any Firefox CSS element. You can declare additional properties in general, additional elements that can be assigned a color, or additional elements that can be images. Any of the items declared in the theme_experiment section of the manifest can be used inside the theme declaration in the same manifest file, as if those items were a native part of the WebExtensions theme API.
theme_experiment is available only in the Nightly and Developer editions of Firefox and requires that the ‘extensions.legacy.enabled’ preference be set to true. And while it also requires more detailed knowledge of Firefox, it essentially gives authors the power to completely theme nearly every aspect of the Firefox user interface.
Hm. Falls ich das richtig verstehe, wird die userChrome.css in gewisser Weise überflüssig. Aber wahrscheinlich habe ich wieder irgendetwas überlesen oder missverstanden. Und das verlinkte bugzilla ticket bringt mich auch mal wieder nicht weiter..