There are several ways to access the Theme Editor.
Colors & Styles
You can access the theme editor in the following way:
From the main menu, click on Marketing » Gallery Colors & Styles.
Click the Create & Customize Your Themes Here link, which is found underneath the Save Your Theme Selection button.
Gallery Settings
You can also access the Theme Editor from the Gallery Settings page for a gallery.
From the main menu, click on Photos » Galleries.
Click on the name of a gallery.
In the Basics section, scroll to the Gallery Theme option and click on Edit Themes.
Gallery Type Settings
You can also access the Theme Editor from the Gallery Types section.
From the main menu, click on Photos » Gallery Types.
Click on the name of a gallery type.
In the Basics section, locate Default Theme and click on Edit Themes.
Client Galleries
You can also access the Theme Editor through a little trick done in the client galleries themselves:
Visit and enter any of your client galleries.
Click and hold your mouse button on the icon with three horizontal lines on the far right.
Continue to hold the mouse button down for ~8 seconds.
The page will refresh in Theme Editor mode. You may release the mouse button at that point.
You can learn more about the Theme Editor in this article.