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It's literally called "windows decorations" in the settings. https://userbase.kde.org/System_Settings/Windows_Decorations

On my system, if you type "window borders" into krunner, it pops up the "Window Decorations" settings page. Krunner can be spooky sometimes.

Krunner is my friend! ;) Between Krunner and a few custom hot-keys I seldom use a conventional app launcher any more...

Thank you for that! Was disabled (so very light grey) because "Use Theme border size" was selected, so I never noticed the whole thing.

I say again, though: Defaults matter!