I was actually searching for a theme like this just a few minutes ago but didn't find anything, so thanks for sharing. Does this use the GTK 4/Libadwaita style?

the reason why people dislike huge margins and rounded corners is because they grew up in the oldschool era of computing (say from late 90s to maybe early-late 2000s). UI back then was designed to be relatively compact and be readable, everything useful is at a glance and it's primarily designed for a keyboard and mouse, so if ther's any margins it's bound to be at least a couple to few pixels at most.

this kinda clashes with the more modern age where designs are a bit more simplified and spaced out (i guess inspiration came from mobile phone design, idk), and text is mostly discarded for more visual design, which if you know what the icons look like it can be a bit more simplistic, but when in 115 there's a small little cloud with an arrow as the get messages button, yeah it's a bit abstract (and now a bit harder to get to that button), meanwhile the new message button is more or less in the spotlight. it's inconsistent imo.

i think it would have been more successful if they stuck to the pre-115 design but just touched it up a bit, maybe get some more modern icons for it and make it feel a it more sleek but without changing the overall layout and design.

and rounded corners are a taste thing, some people might like very slightly rounded corners while very rounded corners just aren't their thing. (i'm one of those people, i just like corners that are like 1-3px rounded, 10 to 20 and above is a bit excessive and i generally associate overly rounded corners with the likes of google and microsoft with their current products).

and this is coming from a gen X lol, i just grew up XP what can i say. although i do like flat design when it's done well (discord gets it right, excluding some rebranding choices).