Initially when you create a project you add a template. These templates come preloaded with one theme style. Once your project opens in the editor you are able to change the themes, which changes all of the styles of your project. To change a theme go to 'Themes' in the sidebar. Once you click on that it will open the entire list of themes. You can scroll through the list to find one that you like. To activate a theme click on the thumbnail of that theme. You can also test several themes by clicking on the thumbnails one at a time. Make sure to wait until it loads before clicking on the next one. Below you will see some of the options available.

This is the Superhero theme.

This is the Public Theme.

This is the Journal theme.