@RootMoose @kde Have you looked at Klassy as a possible solution? It's a theme utility that has quite a few options, including window outlines. You can set it to look exactly like the standard Breeze theme, just with a outline.
GitHub - paulmcauley/klassy: Klassy offers highly customizable theming for the KDE Plasma desktop.
Klassy offers highly customizable theming for the KDE Plasma desktop. - paulmcauley/klassy
GitHub (github.com)