KDE let's me setup myy font sizes, I always use huge fonts and I hate with a passion when devs pixel allign things and break the GUI when I use larger fonts.

I also get exasperated when I get asked to pixel perfect align checkboxes and the labels in web pages, each browser has different checkbox dimensions and styles, and aligning them how the designer wants means a lot of extra styling that will break on users that don't use the defaults(but the designerr wants his pixel aligned stuff on his Mac screen... also rewording messages to align in message boxes or labels

I agree. That said I switched to KDE a few weeks ago and turned into a fan boy during the configuration process. I'm very happy with my current configuration (probably a mix of it actually being good and the Ikea effect). I love that you can just configure almost anything almost exactly to your liking with not all that much effort.

I'm not a huge fan of the default configuration, but the ability to completely change it (with the many panel options) is just amazing (kind of contrary to GNOME).

I think I have figured out at least part of the reason for the spacings always feeling just a bit (or very) off (although I think it's bearable with my theme):

Their Kirigami design support library has a set of pre defined spacings and sizes developers are advised to rigidly stick to. These probably break down in many contexts and might not even be chosen well in the first place.

Another reason might be that KDE doesn't have a design group that is as strong as Gnome's and KDE might not be organized top-down as much (which is probably related to financing).

I think you can fix or mitigate well a lot of the mistakes (Qt is great) by using a good Qt curve theme.

You're correct that KDE's design group isn't organized from the top down. While that might be a problem for a company, I think it's actually a good thing for a community of volunteers because communities need to grow. Everyone has their own lives outside of the work they do for KDE, so it's important that the work load is spread across many people. Anyone can start start designing for KDE, you just need to talk to other KDE designers and submit patches.

I am a fairly new KDE designer who started contributing icons in the spring of 2018 because it was easy and fun.

Glad I’m not alone, KDE’s design system (or lack of such) has always been like nails on a chalkboard for me. GNOME takes it a bit too far in the other direction but is more tolerable (especially with a compact theme). XFCE is probably my favorite and because it strikes a great balance between design, readability, and info density.

Xfce is great, and by far my favourite. But it's harder to sell to the average user because it looks so much older than windows and Mac. At least by default

On the other hand, one thing they do really well is theme support and configurability. I agree with you that out of the box it's pretty bad and they need an real designer to redo Breeze, but themes are allowed to adjust margins and it's possible to make KDE look really nice.

Personally I find that using Source Sans Pro as the font and the Arc theme for most of the rest works well.

What do you use that respects these principles?

Not trying to be snarky but MacOS.

That being said, on Linux I think Elementary OS is at least trying to create a coherent visual design language. Maybe there are others?

Most GTK based DEs are pretty decent in this regard (GNOME, XFCE, Cinnamon, Budgie). Qt based DEs tend to be more utilitarian and weaker when it comes to design.

I apologize, but I don't understand how anything you wrote in any way responds to what I asked, who I asked it to, or could be considered "helpful" in any way.

Please don't inject meaningless flag waving platform propaganda into genuine conversations.

Pretty sure they're just pointing out the polished typography, padding, and alignment of MacOS, which is what the GP's comment was about.