Cocoa made a great set of tutorials on how to customize your blog. I used those and a linkware webset from Touch a Memory to pretty up my blog a bit.
Here are the links to Cocoa's tutorials so far:
Part 1: Choosing colors & patterns
Part 2: HTML code
Part 3: Fonts
Part 4: New Headers (I skipped this because my webset came with one.)
Part 5: Columns & Margins
Kudos to Cocoa for the great tutorials!