Changing themes requires root access / rd mode. And it is shaky stuff, so take warnings seriously.
I have tested the theme creator with default settings and it worked just fine. The most volatile part is using the usb cable to transfer the theme to your 770. If anything goes wrong with that, then you probably need to reflash.
But anyway, if anyone of you is feeling artistic, then, go ahead and make a new theme. If it's cool, share it with us!
Theme maker is missing quite a few maemo theming features, but once you ask for me for them, I'll try to add them as time goes by. At the current state, you can modify pretty much all windowing parts.
Go grab it while it's hot! Oh. Requires java 5 on your computer. Tested only on windows, but I see no reason why it wouldn't work on osx and linuz as well.
I was finally adventureus enough yesterday to test making a theme with the theme maker and made a itunes inspired theme in a few hours. If you're interested, have a look at the blog describing it:
I really like ithe nuvo theme. i might just keep it for a while. once again konttori comes out with a useful, elegant enhancement to the 770. is this theme inspired by os x?
You could have selection of different theme colours, edges/corners on bars, textures for bars, gradients, etc. in the first phase.
The first phase could be implemented with alternative software (scripting Gimp?), it would give more power for us who can program, and it would allow people with little Gimp skills to create their own themes or just prototype a theme idea.