With 6.0 we have started to use SVG files to draw the app icons.  To add a new, custom icon, people must add the SVG file(s) to src/main/resources/VAADIN/appicons in a Maven module. SVG icons added to ${magnolia.resources.dir}/VAADIN/appicons are not found by Vaadin.

Users now can develop a custom content app completely within a light module (besides they need custom java classes). We should give the users the possibility to provide their own, custom SVG icons with light modules. 
 
Since the app icon now must be an SVG icon, if you create a custom app, you have these possibilities: 

1. Rely on the default icon.
2. Reuse an SVG icon which is already in the bundle.
3. Supply a custom icon via Maven module.

 
For (2) it could be helpful to have somewhere a list of all SVG icons which can be used for an app icon! (While using the icon-font, it was a bit easier since you could watch all available icons from the icon font via docu.)
The SVG icons are in a module which belongs to the "internal" repository, hence it is really challenging to find the list of the available SVGs.
=> https://git.magnolia-cms.com/projects/INTERNAL/repos/magnolia-icons/browse/src/main/resources/VAADIN/appicons

Note that Magnolia cloud users with a default subscription package cannot add Maven modules!