I only wanted to make a few small changes on my Movabletype Blog but it wasn't too bad. I did install the plug-in StyleCatcher which gives one click access to installing more templates and then changed a few of the image files.
http://www.sixapart.com/movabletype/styles/
I'm doing another Blog for someone else and we do want to modify the a lot more then what is available from their templates. This looks like it should work real well but I've only played with it a bit.
http://styles.movalog.com/generator/generator.php