Microsoft Windows is abysmal on this front - I have to use Microsoft Windows 10 periodically, while most of my life is spent in KDE, and the disjoint is jarring.
In KDE I can precisely control and set window border thickness and colour.
In Microsoft Windows 10 (and I assume 11 has continued the trend at making this even worse) I can enable a very thin border, of a themed colour, and that border disappears when the window is no longer in focus. This means a couple of terminal windows (eg gitbash) have near impossible to discern edges when they're overlapping.
On top of this, some Microsoft applications seem to break their own styling rules (Office apps, say) which I suspect is the kind of issue that stalled your project.