Hi

As a first step O recommend that you reverse those about:config prefs. They may not be needed for this.

If you open your Firefox Sync settings, do you have add-ons set to sync?

Yes, I have everything checked except credit cards and open tabs.

I have created brand new profiles using the profile manager and activated Firefox Sync in each one and so far it seems the theme isn't changed whenever Sync runs.

I have a feeling my problem was due to how I created the profiles before: after creating a new profile, I copied the content of my main profile folder under AppData\Roaming\Mozilla\Firefox\Profiles to the new profile folder, as I wanted to keep the same toolbar layout. Maybe this messed things up ? What's the best way to duplicate a profile then ?

Well, it seems I spoke too soon, the default grey theme got applied on all profiles again... Really frustrating !

Setting services.sync.prefs.sync.extensions.activeThemeID = false should be sufficient to exclude the theme from sync (the other pref is no longer supported). You can also try this pref in addition:

• services.sync.addons.ignoreUserEnabledChanges = true

I think that last setting made the trick : the enabled/disabled status of the themes aren't synced between profiles anymore, and as result, each profile keeps its active theme even after a sync. Thanks !