One of the things I gave up in moving from WordPress classic to WordPress.com is theme editing. My previous blog had its own theme, or rather, user-switchable family of themes.
I’ve started off here with the Simpla theme. I’ve customized the CSS a little; for example, I’ve floated images at the right of posts. I’ve used widgets to build a different sidebar.
Lovely though Simpla is, I think I’ll be using a certain different theme before too long. The Sandbox is a theme for themers.
Be sure to share your CSS code with others in the forum. They will appreciate it, and this will probably drive much traffic to your blog anyway.
Good luck!