The “Active Theme” framework seems very lacking at best, if not almost completely broken at worst. Sure, you can export themes but you cannot import them back, nor the engine recognizes your previously exported .json files as existing themes to let you pick from them, essentially reverting you back to default “Dark” and removing everything else, no matter what you left it on.
The color list entry naming is also very vague and hard to see what each does, to the point where someone took it upon themselves to make a page that helps you with this.
The way various elements are linked now also kinda invalidates certain color schemes, say if you want to replicate the look of UE4, you end up with white-on-white/black-on-black regions more often than not for certain UI elements.
I really hope this “mechanic” gets properly implemented.