If themes (26, including “default”) haven’t yet arrived as an option in your gMail account, check out the screenshots on Flickr!
You’ll know when you’ve “got themes” because your gMail account will look like this:
Here’s how to configure the themes, once they’re available.
First, go to “Settings” on the upper right task bar. Then click “Themes” which will be on the far right of the “General Settings” menu bar. Click a theme and it’s automatically activated. Be sure to scroll to the bottom of the page! Some designs are more clever on the bottom than on the top.