it's very easy! If you come up with something that you want to share, or want to submit for inclusion as a system style or theme, just post to the comm and follow the posting guidelines. You'll need to do a one-time submission of a licensing agreement (it doesn't sign over the rights to anything you produce, just gives us a permanent license to use, re-distribute, and re-license anything you submit to us) so that we're legally in the clear with using your contributions and you're explicitly protected too (by a statement that you retain full rights to any designs/work/etc, and by a statement that we don't expect you to support anything you make and don't hold you accountable for any future problems with the contribution). once you've got that done, it's just "make theme or style, post theme or style to dreamscapes", and the magic pixies (okay, usually ninetydegrees) make it work.