O hai, I'm the one who made the existing Gurochan styles for kokonotsuba

The one called "Gurochan" (currently the default) was my attempt at emulating the original Wakaba Gurochan style as closely as possible, just for the sake of having those styles on Heyuri. This was long before the plans to move Gurochan back to kokonotsuba were established and I was working on a ton of different styles all at once, so I didn't rigorously test it on mobile or anything ^^;

It uses a sans-serif font family at 10pt - traditionally, 10pt was a common size for sans-serif fonts because they would appear much larger than serif fonts at the same point size on Windows. On mobile devices, serif and sans-serif fonts are usually a similar size to each other at the same point size, so 10pt ends up being too small for comfort. You can get around this by making an override in the CSS for mobile display resolutions, but I had neglected to add such a thing to the Gurochan style. Something like this should correct it (you can paste this into the custom CSS area in Settings -> Style):

@media screen and (max-width: 650px) {
	body {
		font-size: 12pt;
	}
}

"Guro Light" (formerly called Gurochan 2021 since I just assumed that's when it was made) was something I threw together as fast as possible once it became apparent that a software switch was imminent. However, I'd never officially "finished" it - it was just taken from my test kokonotsuba instance in the WIP state I'd left it in. I think it's mostly done, but there may be areas I've neglected

Regarding the Dark version: I haven't even started on it, mainly because I fell ill while I was working on the Light version. It's gonna be a little difficult now that the transition has already occurred - and truthfully, I've just not been in the right mood to work on it ^^;