Two other tabs you'll notice in the Theme Editor are "colors & fonts" and "backgrounds." These are the options that allow you to customize a theme's default colors and background. (Click images to enlarge.)
Briefly, the "color & theme" tab of your Theme Editor allows you to make global changes to the way navigation text, body text, title text and hyperlinks appear on your website. This is done with a color palette tool that launches for each option.
With the "backgrounds" tab in the Theme Editor, you can select from a collection of pattens and textures to customize the tiling of your site's background. You can also upload your own images for use as a background.