Check the wiki - there should be some sort of theme packager application available, but I haven't tried it since Diablo/N810 (I'm not even sure if it's the same app). You should be able to get your hands dirty with it without too much effort, I think. There is no coding involved, but there is a configuration file that you will have to modify, which requires some technical competence. As I said, though, there should be some tutorials and documentation online, and you're better off fiddling with some GTK themes from http://gnome-look.org on a desktop machine first. If you've ever worked with something like CSS, then the GTK config files shouldn't be a problem. (They're not really that similar, but in terms of complexity, it seemed a decent example.)