Of the three (default, Photon, Alexandria), Photon is my favourite by a long way.
I use dark mode on everything so I have that bias. I find Photon easier to read than the others site to the colors.
The larger image preview is easier to see. I prefer text-only in general, but I notice my eyes are less strained trying to make out thumbnail details with Photon.
The UI layout seems tidier too.
Is the long term plan to keep them optional or phase out some in favour of others?