Is there anywhere I can see a complete list of all themes with pictures? I've seen this one, but it doesn't seem to include everything.

This whole thing started with me trying to find out what the Sacred Spellbook Lighira looks like, because there don't seem to be any in existence in the whole game and they're not on the above list, but they're an option under Lighira themes in the advanced search.

to be fair, the sacred spellbook ligh was literally released like. yesterday

For theme guides
Shadowized's guide (currently up to date as of this past summerfest, and does not yet have the new lg themes)

seasonally guides are pinned in the pet design forum (though there's now the in-game official page of themes for a fest, the pinned threads include the original designer)

the new themes released with the new section of the lost grove are currently displayed in the first post of the thread announcing the release of the new section (it was updated to include them, so if you checked the ping for the thread and they weren't there then, it's because Krin hadn't updated with them yet)
you can also see them listed with their designer in this thread