"Breeze-themed windows now have a subtle outline around them, which not only looks classy."

Really? You think that's classy?
Please explain to me how an abundance of thin black lines on a light background becomes classy?
What functionality does it have? What aesthetics, beauty, need for it?

On the contrary, it looks superfluous.
In terms of design, it's a disaster! I'm telling you as a designer with 23 years of experience. As the art director of a strong design studio. It's the wrong decision.
The thin black lines on the edges of all the windows strip the screen and irritate the eye. It's like the eyes are being cut by thin knife blades...
These black lines are unnecessary in light interfaces. At all. At all.
Look how messy the designer's workspace has become:
https://i.imgur.com/ZbibFeR.jpg
And how clean it used to look:
https://i.imgur.com/cvDpaIU.png

Maybe it makes a difference for a dark interface. I don't know, I prepare images for print catalogs and for websites, all of which have white backgrounds, so I work with light interfaces.

If you're going to introduce such a significant visual feature, give you the ability to control it, up to and including the ability to turn it off.

This is K D E !
It's the most comfortable environment for designers. And suddenly there's a visual bug.

I'll add. The windows with this solution began to look outdated. It's like Windows 95.

A huge, huge step backwards in KDE design. Epic mistake. Not inherent to KDE.

Yes, the world's gone mad. You could use a different window decoration if you want. The little dark border isn't such a big deal. The halfbaked unfinished not working highlight thing they implemented is though. If you use a different decorator like, say, klassy, you're good. But the total inconsitency of that highlight wannabee requires serious editing in color schemes, especially on dark themes. I don't get it why this unfinished scripting is implemented whilst third party stuff already did a much better job at that, both on colors as icons.
Anywhooo..., klassy window decoration, amongst some others, allows you to set the borders. But again, that highlight nonsense destroys a lot so you might have to change the color scheme ( header section) as well. https://imgur.com/a/lDwvYQ9

You used to be able to set frame colors in ~/.config/kdeglobals. Under [WM], you could add frame=given rgb color. With that highlight nonsense, this no longer works when you use default breeze decoration. As you can see in the screenshot, only the frame without the titlebar is affected. You'd need to fiddle some more I'm afraid. https://imgur.com/a/JVgcxWc

Thank you for your time.
Yes, I had to install the "Arc" window design.
Still, I hope my voice will be heard by those responsible for these outlines.

If you want to achieve more in terms of window decoration, an aurorae theme only goes so far. Klassy is an entire window decoration application.https://imgur.com/a/ASwjJL4

You surprised me!
I read this page: https://github.com/paulmcauley/klassy

And it gives a warning at the very beginning about instability. It is not acceptable for me to have an unstable system.

I appreciate your attention for being here in this forum and responding to user questions. Thank you.

Still, I think Breeze is the best window design. Excellent design. I admire it. I enjoy this theme. And that's why I really want it to stay perfect. That is why I was not indifferent and came to this forum.
I really hope the KDE designers will listen to me.

In light themes the dark window outline has no justification in terms of functionality and aesthetics.
There is no true and reliable foundation underneath this solution.

Personal taste. Ever since Lightly was released I've been using it ever since. I don't really need to use klassy, lightly does both application style as window decoration. To me, breeze is like some slightly upgraded windows xp. Screenshot is lightly application and window decoration (hence the roundness in the ui) and rounded Breeze RC Chameleon icons. https://imgur.com/a/GqQ8FYB