geekamp works for winamp5, too.
The only thing that could make your theme not work correctly with winamp5 is if you're manipulating winamp's WindowClass/Caption for the playlist, the media library, etc...
Then, everything depends on the skin you're using:
- if you're using the "classic" skin, all these windows will react as winamp2's windows (WinampPE, etc)
- if you're using some freeform skin, they'll react as winamp3's windows (BaseWindow_RootWnd)