In Atticus you have two main views - the Writing tab, which acts as a word processor, and the Formatting tab, which is where you'll create your custom theme design.
If you need to edit your content, click Writing from the top center and there is a standard toolbar in this section.
If you need to edit the design and formatting, click Formatting from the top center. Then, select your theme, and click the three vertical dots next to the theme title to open the theme editor. Work your way through the options and then save your theme.