Theme on XBOX 360

[Salsa Party Animal]

I have downloaded some themes for XBOX 360 and I can't find a way to use the theme as I can only copy, move, delete not use it. Is this for Windows XP or what ?

[Synthesthesia]

Press the guide button, and it's under the options tab there, I couldn't find it before either, really odd place tbh :/ (or something like options, haven't used my 360 in a while)