Your question is not clear, what do you want to do. Themes allow the change of the overall look and feel of a Moodle installations, e.g. colors icons etc. What is it you want to change?
Some ways I like to make a moodle a little more lively is to add RSS feeds and twitter feeds. It means the site is automagically updated on subjects I think are relevant without me have to create new content. You can see what I have done here
http://www.questiontype.co.uk/mdl/
But it will not win any design or beauty awards in the near future.