Is your phone rooted? If so the best way to install a theme is to download Metamorph(its free unless you want to donate to the dev)then download your theme and put it in the folder called Androidthemes(It doesn't really matter though). Open Metamorph and allow all it says (sometimes it force closes don't worry about that though just reopen it). Then apply the theme and walla! New theme.
Heres my personal favorite:
MM Smoked Glass Framework for 2.1 - Droid Forum - Verizon Droid & the Motorola Droid Forum
Hope it helps.