There are a two ways you can change your theme.
In the editor
In settings
Note: you can only set a custom theme from inside the editor.
In the editor
Create a new Atomic Essay.
Press the palette icon in the top right of the screen.
To set a color theme, browse from a collection of pre-built themes or choose "Custom" to set your own custom theme.
From here you can also set a Font Pairing.
And if you are writing an Atomic Essay, you can also choose the Layout, Font Size, Author, and Footer.
In settings
You also have access to the above options in settings. To get there, press your name in the top left, then choose "Settings" > "Theme". Just note that you can only set a custom theme from inside the editor.