The only thing I can tell you is make sure the theme is for whatever you have installed as a theme changer (Summerboard, etc.). As far as I remember you have to install one before the other and maybe even some additional little apps that will allow you to change the stuff, make sure you read all the info posted in cydia when you download the theme and such they usually give instructions or worthwhile details. Other than what I just said I can't help anymore, sorry.
