The theme dropdown is a list of built in color and font themes that you can assign to your donation page. 

Once you select a theme, you can edit continue to customize the color and font selections using the sub-headers for the Body, Header, Title, Subtitle, Label, and Error Message. For example, to change the background color for the page, click on the Background color box to open a color picker to select your custom color.

Once you have finished updating the colors and fonts in each sub-tab, you can save your theme for future use by clicking on Save Current As. A dialog box will open to allow you to name your theme and click Save to add it to the list of themes available for any page. The custom themes appear at the bottom of the list of themes.