To change the Theme of your OpenScholar site, go into the Admin Bar > Appearance > Themes. Here you can choose from different themes. Some themes even have something called flavors. The design of the page stays the same but either a background color or a background image changes. When you choose a theme you like, click the Save button. You can not edit the HTML or CSS of any Themes.
*Note: When changing your theme, the layout of the last theme is kept, so this may effect the way the site is displayed in a browser. When changing a theme, always keep in mind you may also need to change the layout of your content so it displays as intended.