I hated the UI changes. And it wasn't just a cosmetic standpoint(though, the spellbook absolutely was).
I hated the new scrollbar, I hated how there was a lack of audio feedback for your clicks, I hated how the colors worked(that was a pure amateur job), I hated the excessive and redundant information on the inventory screen....
UI mods only addressed so much of that. Most of them just changed the art, and that was one of the few things I liked. You could also change the positioning and size of different elements of the UI, but those were both non-issues to me. I could get used to that overtime. However, none of my actual complaints were addressed no matter how "customizable" the UI was.
The absolute worst was asking for help and then just being told by the community how "awesome" the new UI was and to just like it. It made me really hate their fanbase. It didn't help with Beamdog burying their head in the sand and refusing to listen the other large portion of the fanbase, like with Trent literally begging the fans to go and post positive reviews on their two major outlets because of all the backlash they were receiving. Just look at the reviews on GoG, for instance.
I don't know what black/red UI you're talking about. However, I did like the art of the black/white UI with the dragon. We might be talking about the same thing, but I don't recall any red.