I initially reported this on the BTS, since I didn't know where to send my feedback. Thanks, Hugo, for pointing me to this forum. I'll pretty much just copy/paste what I wrote on Bug 346917 - breeze-dark lack of contrast between window borders and backgrounds: overlapping windows can get confusing

I love breeze-dark. There are difficulties with some non-KDE apps, especially firefox, but also some other apps where you get stuff like dark blue links on a gray background, but that's not what I want to address here.

One usability issue I have with breeze-dark is that my eye doesn't always see the boundary of a window, if I have two windows overlapping. With the title bar the same color as the backgrounds, one window blends into another visually. If I have overlapping windows, I want to be able to see that, oh, that bit is actually from another window.

IDK if it's possible to choose a window border color that isn't ugly, but isn't quite the same as the backgrounds.

One change I made in System Settings -> Colors -> Colors is:
Active Titlebar: a dark red (#630000)
Inactive Titlebar: a very dark red (#280000)

I use a solid dark blue desktop background (#004), and the gray, dark blue, and dark red actually look ok. The red for active titlebars is bolder than the rest of the color scheme, but I don't find it garish. White text in titlebars is easily readable against the dark red.

I also use the "Dim Inactive" desktop effect, with strength 10. I use "disable compositing for fullscreen windows" so it doesn't affect mpv playing fullscreen video on one monitor, while the focus is on something else. This effect makes inactive titlebars even dimmer, which isn't really needed, but it's ok.

I might turn off that effect now that I've changed the default titlebar colors to something that makes it easier to tell which window has the keyboard focus. (I use focus follows mouse.) IIRC, lack of difference between active/inactive windows was another weak point of the default breeze-dark theme. The shadow effect along the bottom and right sides of a window is enough of a visual cue to avoid having my eye wander out into another window, and the dark-red title bars take care of the top for me.

The left side of a window can still be an issue. (I use firefox with tree-style tabs, and the only way they are usable with breeze-dark is with the "default (speficied by theme)" skin. So the left edge of my firefox windows are flat grey. If I have two FF windows overlapping by just a bit, the tab bar of the top window can visually get mixed up with the tab bar of the window below. I'm finding this less of a problem now that I'm more used to the theme, but I think I'd like the option to have some kind of border at the left edge.

I'm not a very visual person, so I don't have any wonderful ideas. I'm sure getting non-KDE apps to be usable with breeze-dark is a pretty frequent FAQ, but this visual confusion happens with proper KDE/Qt apps, like okular + konsole + dolphin.

Hugo Pereira Da Costa replied to my initial bug report version of this post:
> There also have been discussions there about re-introducing a window "glow" ala oxygen for active window shadow (to address the same issue, in fact)

I copied that in here since it's probably relevant do discussion.

Glow. It's a feature I miss.

With a multiple monitor setup, the "Dim Inactive" isn't very effective, Unless you add an extra option for extra screens.

+1 for the problem. Glow might help but deflattens the design.

Heiko Tietze wrote:+1 for the problem. Glow might help but deflattens the design.

There's already a black shadow. Only if I could turn those shadows into a white would be great.

Stumbled over viewtopic.php?f=285&t=125549 "Glow instead of shadow for the dark breeze theme" but haven't time to read it now.