I see, thanks for the details. All three of those themes are known working themes, so don't worry about trying to remove the plugins. Unfortunately it looks to be some kind of weird driver or Windows bug (if I had to guess it's video drivers). It's strange to hear that you don't have issues with 11.2, but you do with the betas; my guess is that means that there are some bugs with .NET Core that don't exist with the older .NET version (which is probably going to cause us some trouble when the official release goes out).
So, as far as solving it, I would manually run Windows Update to make sure you're 100% up to date, and then look for the latest video drivers and install those to see if that helps. You are on Windows 10, correct?