1. Go to the page you want to add a theme to.
2. Click the Theme Settings Button. If you don't see this then themes may not be active on your site or that page.
3. The theme window opens up.
4. Click the Select Theme Drop down and find a category that works best for you.
5. Once in the category of the theme you want, find the theme that works best for you and click it.
6. You'll notice that the theme will now be displayed in the right hand corner just above the page content. Click update page.
7. Back on the site you'll see the changes are apparent. Themes do not affect sidebars.