Themes allow a user to format an entire document with a certain look. This includes fonts and colors. Editing Themes in Excel can make data easier to read and organize. To edit Styles or Themes, first open a document, then go to Page Layout, and find Themes on the left. By clicking the drop down boxes, you can select pre-made themes, fonts, colors, and effects. You can also create your own Themes and Styles and save them for later if you are logged into you Microsoft Account.
Link to Knowledgebase article: http://helpdesk.etown.edu/helpconsole2012/KB/default.aspx?pageid=styles_and_themes