I tried it out now, and it seems not to be photon styled. Also, in the ‘latest downloads’ example, if you do not set ‘overflow: hidden’ (at least I) get a scroll horizontal scroll bar. Not exactly what I would expect from default styles.
For my Add-on I tried to (kinda) follow the spec (and tried to implement it for my own). Also, I could not find any information on headings in wing panels on the Photon site?
This is how my popup look now:
I think Photon look really great, and if I can help in any way to make it easier for WebExtensions to implement it, I am glad to help