Theme? There are all kinds of themes/settings for RPGs, from fantasy to sci-fi-to steampunk to generic urban role-playing.
EDIT:
Darths & Droids is one of the most helpful resources I've ever used to understand the way to play RPGs; it's a webcomic with a storyline that the Star Wars movies is actually the RPG campaign of a group of gamers. Yes, it's a webcomic, and yes, it does satirize RPG culture to a degree. But that gives me valuable insight into said culture, and the way the GM of the story really does set a fine example of how to run an RPG. It also attempts to explain unfamiliar RPG jargon in a way that non-gamers will understand, which is also quite helpful. Link here:
http://www.darthsanddroids.net/episodes/0001.htmlAs an added bonus, it actually makes more sense than the Star Wars prequel movies; case in point: one of the most memorable lines is, "Jar Jar, you're a genius!" And it's actually true.
Also: why don't our RPGs have more player-generated plans like this?
http://www.darthsanddroids.net/episodes/0098.html