Created attachment 132734 [details]
The screenshot shows the incorrect display of window control icons (close, minimize and restore) in Firefox.

STEPS TO REPRODUCE
1. "System Settings" -> "Global Theme" -> "Breeze Dark" or "Sweet".
2. "System Settings" -> "Application Style" -> "Application Style" -> "Configure GNOME/GTK Application Style..." -> "GTK theme" -> "McMojave-dark" (https://store.kde.org/p/1275087) or "Sweet Dark" (https://store.kde.org/p/1253385).
3. Reboot your system.
4. Start for example Firefox (with already disabled "Menu" -> "Customize..." -> "Title Bar").

OBSERVED RESULT
*See screenshot.*

EXPECTED RESULT
Correct display of window control icons (close, minimize and restore) in accordance with the GTK theme.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Arch Linux.
KDE Plasma Version: 5.20.1.
KDE Frameworks Version: 5.75.0.
Qt Version: 5.15.1.

ADDITIONAL INFORMATION
I've only tested this with Firefox.  This probably applies to all GTK CSD applications.
With the GTK theme "Breeze" the window control icons used are correct but larger than in other applications.  Without rebooting and with GTK themes "Sweet Dark" or "Mojave-dark" the window control icons are correct but they are smaller than in other applications.