Hi ndmccain,
It’s a bit tricky to spot the custom overlay settings. Once you have activated the theme and selected “custom” overlay, you need to reload the page, go back down to themes and now you should see “Modern 2020 Theme Customizer” at the bottom to the Themes section of the navigation.
You can change icons for forums under “Forums” -> “Manage Groups And Forums” by clicking the edit button for the forum you want to change.
A bit further down is an area where you can select icons for different purposes.
To add to the selectable icons you can upload individual icons under “Forum” -> “Custom Icons” and manage sets of icons under “Options” -> “Icon sets”.
We are gradually simplifying the Admin panel, but we still have some work to do. Please let me know if you have any other questions.
/Marcus @ Simple:Press