It seems I may be in the minority, but I generally like the look of XF2. I don't like the blackened themes here because they look like they could be default templates for an Invision Board forum from 2008. That is to say, outdated and ugly as hell. Granted, that might be a pinch of hyperbole on my part. Most of the other options are full-width styles, and I actually prefer the empty spaces on the side. On that note, will the final version retain the change width button? I only had a chance to check mobile before the default width was changed, but now that I'm on desktop I don't see the Change Width text to make it smaller again. Somewhat related: is there no way to hide the sidebar?
The only style I really use currently is the UI.X Dark style specifically because I feel it most closely matches the "modern" designs that are so common in XF2. My only real gripe about it is that the UI.X styles don't display the tags of a thread, but that's not a problem in the XF2 styles, so I'm already pretty satisfied all things considered. On the topic of tags, I do kinda feel like their inclusion in the thread list on mobile makes the thread rows a bit more elongated than I'd like, but the benefit of seeing the first few tags might outweigh that.
I do agree with the others that giving the dark style a splash of color would be an improvement. Some have mentioned darkening the gray, but I'd personally be fine whether the current gray is kept or it's made darker. I think an accent color would go much further aesthetically.
As a request, the UI.X styles have floating to-top and to-bottom buttons so you can quickly navigate to either end no matter where you are on the page. The XF2 styles have a button to go to the top on mobile, and that is probably the more useful of the two, but would it be possible to also add a button to go to the bottom?
Lastly, yay, centered text retains formatting when pasted over. This is gonna be huge for my third post in 6 years.