Okay, here's a theme in installable form, plus a new version of the client:
Client:
http://faforever.com/faf/FAForeverLobby-376.msiPlease note this is for reference use by theme creators, this may crash hard depending on what you try! :] The theme will also instantly break as soon as a different client version is installed (it will refuse to load it, safely). Later versions, probably starting with 0.7 or so, will use a special UI versioning scheme so themes can stay valid for more patch levels. There are simply too many tricky UI changes going on while we are in beta so I have to invalidate themes every time or risk locking a lot of people out by accident.
Good news: I also added a "Reload Stylesheet" option to the theme menu so you can quickly check most changes while editing your stylesheet (no reconnect needed).
Caveat for theme creators: If you look into the client.css file of the sample installer of ozonex's UEF Theme I added, you will see a placeholder named "%THEMEPATH%" used in image urls, that's important and points to the directory your theme was loaded from (so people can rename your themes and qt knows where to find your images). Don't hardcode the paths in there please (themes might fail terribly on windows XP otherwise).
Speaking of WindowsXP, I need a volunteer to test this on XP for me.