To chime in on the old fart front, here's my gripes. Apply to both the light and dark themes (although I'm using the light theme because the explosion of colour on the dark one was driving me nuts).
* Whitespace. Whitespace everywhere! Not sure if this is catering to phones or UI fashion but it looks bloody awful on my desktop. Many elements and the spacing between them seem oversized, especially what seems to be called the "reactions bar" (I never use it so it's gone into a ublock rule now).
* Much enhugened typeface now. I've got them set to small and they still look too big to me.
* Horrible rounded corners everywhere. Yes, the old theme had roundrects as well but they were much less circular than this.
* Lots of low-or-no-contrast all over the place. Less clear demarcation between many elements, the old theme had fairly solid darkish-grey lines
* Conversely, high contrast where it isn't needed. The rainbow of avatar colours is especially distracting especially since they're not actually drawing your attention to anything important. The old monochrome avatar image was better
* As previously mentioned, no forum navbar at the bottom of each page
* Not sure what's going on with it, but I also see no differentiation between read and unread posts - unless the unread ones are somehow just more spidery or something. Possibly this effect is reliant on loading remote fonts, which I keep disabled.
* Colour of selected texts now seems to follow the StH theme colours and not the colour I have set. Jarring. Even more annoyingly, it's the same colour as the blue UI elements in light mode but the same blue colour in dark mode. Even more jarring.
* Not really anything to do with the theme as the old site had the same problem, but the forum still doesn't seem to be able to distinguish between forums you've looked at. Forums show as having "new" posts if there's any unread posts in them at all, not just since your last visit, so you have to compulsively go "mark forum read" every five minutes to get the new posts icon to mean anything.
All £0.02, YMMV, IMHO, etc. I wouldn't go so far to call it a forum killer but to my aged sensibilities it doesn't look anywhere near as professional as the prior design. And yes, Windows 2000
was the best windows GUI since you ask!
* yells at cloud *