Please add a dedicated option in Firefox Sync to control theme syncing independently of extensions and other settings.
Users often want their bookmarks, passwords, and extensions synchronized across computers while choosing a different theme for each device. A theme might match the operating system’s appearance, suit a particular display, or help distinguish a work computer from a personal one.
Keeping themes separate currently involves obscure about(:)config preferences, and the workaround can also affect how extension enabled/disabled states sync. This should be a straightforward choice in the normal Sync settings.
A single “Themes” checkbox would make this easier to discover and use. With it unchecked on a device, that device’s theme selection should remain local without affecting any other synchronization preferences.