The theme module allows you to choose which theme you'd like to be used on the front end of your website.
Selecting a Theme
To choose a theme, go into Design → Themes and select the theme that you'd like to use, and click "Save".
You can see an image of what the theme looks like by clicking Preview.
Many themes have theme options which control certain aspects of the display and behavior of themes. If a theme supports theme options, you'll see an Options link next to the them. Clicking on it takes you to a theme options page.