Yes, this is already available. Check out the LaunchBox\StartupThemes folder.
I've heard this from one other person, but I haven't been able to replicate it as of yet. I think it must be a particular system configuration or something. For now, you might just have to turn off startup screens until we can get it figured out.
I can't predict all of people's hacks that are in place, so I can't make every situation just magically work perfectly. Startup screens are a complicated mess of a feature, pretty much no matter what we do. I've done my best to make them work generically while not requiring a bunch of tinkering (and we've honestly made great strides in this area compared to previously available solutions), but there's no promises that everything will work perfectly in every situation. That said, I do have literally every emulator on my systems working perfectly with the startup screens, so they do work well with most configurations, and that was my goal.
Merry Christmas everybody.