If it's hard for you to create music that fits a theme, then maybe you should create some songs and then try to see what theme you could match them to.
As far as a list of music themes which would be really awesome (for me, as I'd use them in my game):
"Exploration" music, for when the player is out adventuring in the wilderness
"Lost in the woods" music, spooky-ish music for when you are deep in the woods and you aren't sure how to get back out
"Battle" music, fast-paced music played while fighting, also need one for "boss" type batlles
"Dungeon-crawler" music, for when you are making your way through dank, cavernous dungeons
Buying/Selling Items music
"Wandering through town" music
Title screen music
Anything else you come up with
But really, you don't need to listen to my ideas. Just make something that sounds good to you and that you like.