You can launch the Theme Editor by clicking the "theme" button at the top of your SiteBuilder. At the bottom of the screen is a theme chooser that also shows you which theme you currently have applied.
Use the scrollbar at the bottom to see what themes are available and click on a thumbnail to preview how your site would look with the new theme applied. Once you decide on a theme, click the green "apply" button to set your theme. You'll see the following message:
"Your theme changes have been applied! Remember to publish your site so visitors see these changes."
Click the green "publish" button to publish your changes to your live site. Once you do this, you'll be asked if you want to "return to builder" or "customize theme."