From The FunKey Wiki
As announced in the previous section, a great new feature introduced in the latest firmware update is the ability to create and add additional RetroFE themes to the FunKey S! Alongside this new ability are two new themes: EmuStation and RetroDex, with the latter being created by community member Siryatan. New themes are placed in the /FunKey/.retrofe/layouts/ location of the FunKey S filesystem, with the layouts.list folder in that folder updated to include the name of the new theme. As a guide to those looking to create their own themes, to compliment the existing tutorial on the RetroFE website, the layout.xml files of the two new themes are full of comments explaining each section.