On desktop, it should be possible to disable syncing themes, or at least stop forcing the same theme everywhere. All other synced things are related to productivity but themes are something different. You want to have your bookmarks, history, passwords, extensions etc. the same on all devices, but you might want to have different themes on desktop and laptop eg. because of the difference in screen size.
I'm submitting this exactly because of that. I use Firefox on Kubuntu and on my desktop I use the Breeze theme in the entire system but on my laptop I have the Aritim theme and if syncing extensions is on, one of the themes is forced across both devices.
Because of that a user is left with the choice: either all extensions get synced but has to use the same theme everywhere on Firefox and the entire OS, or it's possible to use different themes but extensions don't get synced. This is a very unnecessary obstacle.
Thanks for submitting an idea to the Mozilla Connect community! Your idea is now open to votes (aka kudos) and comments.