Atlas comes with a complete selection of Google Fonts allowing you to customize the Header and Body fonts respectively. It also comes with a number of color options.
To access the Customizer via the WordPress Admin Panel, click on Appearance and then Customize.
To access the Customizer from the front end of your site, again, when logged in as an administrator, click on the Customize link in the WordPress Admin Bar.
Once the Customizer has loaded you will see a Sidebar on the left with some tabs, click Atlas Theme Options > Appearance Options.
Finally, you will see two drop-down menus and some color selectors where you can select your fonts and colors.