Forum response -Settings for themes

Settings for themes will default back and will need to be re-enabled:

1. Visit https://addons.mozilla.org/firefox/themes/.
2. Click on the theme you want to install.
3. Click the Install theme button. Click Remove if you change your mind. Your changes will apply automatically.

See Use themes to change the look of Firefox or Built-in themes in Firefox - alternative to complete themes to learn more about themes.