I am on 94.0.2 too. And as you can see from my screenshot the frame is skin. But the menu is not skin. Maybe you can see if rebooting helps. or try uninstall and reinstalling Windowblinds.
I remember, my Firefox was not skinned few months back too. But, somehow, not sure what happened, it get skin itself one day and it stay skinned since then.
Thank you,
Basj,
Stardock Community Assistant