Quote:Mozilla released Firefox Color in early June 2018 to the public. The Test Pilot Experiment enables Firefox users to change the appearance of interface elements of the browser.
Firefox users needed to install the Firefox Color extension in the browser to get started. The configuration itself happens on the official Firefox Color website where users could select one of the preset themes or create a custom theme for the browser instead.
Theme creation was a matter of selecting one of the supported interface elements and a color or texture for that interface element. Firefox would be updated automatically to reflect the change, and users of Firefox Color could make the custom theme the default in the browser or share it with others.

Full reading: https://www.ghacks.net/2018/10/03/style-firefox-with-firefox-color-version-2/