It is possible you selected one of the Colorways themes that are included in Firefox. You can find the built-in themes (light, dark, system) and further down the Colorways themes included in Firefox on the "Add-ons and Themes" page (Ctrl+Shift+A; Themes tab) and select a theme you like. *https://support.mozilla.org/en-US/kb/use-themes-change-look-of-firefox *https://support.mozilla.org/en-US/kb/personalize-firefox-colorways