*Update* I finally found out how to do this. If you open up the Launcher and click Options, on the left click the Homecoming section and type in the extra launch parameters line -cov for the City of Villains login screen and -uiskin 2 for Going Rogue login screen. Leave that line blank again if you want to go back to the City of Heroes login screen. Hopes this helps some people!
I second this because I, too, am having trouble figuring out how to switch the themes for the login screen. I think I found where you type in the "uiskin 2" line or whichever you prefer but it's still not changing it from the City of Heroes login screen. Also it's been a couple of years since I've messed with a computer itself so I feel pretty new at this again.
