When you first fire up Word 2013, you’ll notice that on the Design tab, in the Themes gallery, you have a set of 10 themes available. You aren’t limited to these themes, however; you can also create your own themes, find additional themes on Office.com, or even go back to the original theme used on the template you started with. (Assuming, of course, you started with a template.)
Open the document with which you want to work.
Click the Design tab.
The themes palette appears. The selected theme is shaded blue.
Click the theme that you want to apply to the document. Depending on the theme you select, you might see the font change slightly as well as the colors and effects ...