Select the Style tab from your left sidebar in the website editor.

Click on the Theme section to expand and view ready-made themes. The default view will show all themes.

To narrow your selection by genre, click the dropdown labeled Theme Category. You can choose from some of the most common book genres.

Your current theme will be highlighted with a red background. When you click a different theme, the highlight will move to the new theme. 

Your new theme will not show in the preview section until you save the change. To view your site with the new theme, click the blue Save button.

This will cause a popup box to appear. Briefly describe your changes and click OK. 

A confirmation popup will appear. Click OK again. 

The final step is to refresh your page. Click your browser's reload button beside your address bar or press ⌘ Command + R (mac) or Ctrl + F5 (windows) on your keyboard.

Your website editor will reload showing the new theme.