To give your onepager a unique design quickly you can utilize the Theme options available inside the Builder. 

To access the Theme options open any onepager inside the Builder. You will find a button in the top banner that says "Design" and will open a panel on the right side. At the top you will see a button that says "Pre-Made Themes" like in the following screenshot:

You will then be able to see a growing library of the Themes to select from. You can select from Light Themes, Dark Themes and Graphic Themes.

It is important to note that selecting a theme will overwrite some design changes that you might have made. It is recommended to start exploring the Themes during the time you are building out your onepager.