Here you can customize the header background color, the header text color, and either a light or dark color scheme.
One important thing to note is that for AMP pages to properly validate, your site must have a site icon image. If you don’t already have one, you can add a site icon on the Design > Customize page in the “Site Title, Tagline, and Logo” section.
Another thing to note is that if you have any imported listings, they will need to have an author assigned to properly validate. You can do this using the Bulk Edit feature by selecting the checkbox at the top of the list view (Content > Listings > View All Listings), then in the Bulk Actions dropdown select Edit, then Apply. Then in the edit fields that show up select the author you want to assign the listings to and click Update.
You can also choose a default author to automatically assign to newly imported listings going forward by clicking Content > Listings > Settings and in the IDX Imported Listings section, choose the default author and Save.
Last thing to note is only Posts and Listings are AMP eligible. Pages are not AMP eligible at this time.