The following guidelines are not required for submission to Flathub, but are best practices we recommend and consider for curation and promotion. These guidelines are designed to help your app and Flathub itself appear in the best light to users—they may change over time based on developer and user feedback.
I do think that as a store it should have some guidelines on quality, but not of the application icon’s styling
Realistically if you have light and dark you should include a screenshot of both.