Click on the "Theme & Preview" button on the top right, and it will open the "Theming Options" page.
On the left side, you will see the theming options for Fonts and Colors (shown below):
You will see three options to customize the colors:
- Accent Color (i.e. your signature brand color)
- Primary Color (i.e. font color on menus)
- Secondary Color (i.e. background color on menus)