With Cinnamon I have all windows themed dark as they should be.

But as soon as I switch to KDE on the same PC -- some minor windows and apps are using light themes, even though GTK3 and GTK2 dark themes are applied in settings/Gnome application style. How to make everything dark?

Please see screenshots. First two - Cinnamon, last two - KDE with breezer dark and adwaita-dark for gtk.

https://www.dropbox.com/sh/v48z8uif8jm2bcv/AAAYck0WGdzGZTl8xPSevaQya?dl=0

What version of KDE and linux distro are you using?

I assume that you've used KDE's System Settings to change the applications themes to dark (for the Application Theme (for KDE/QT apps), Desktop Theme, Color Scheme and GTK Theme).
In KDE's System Settings, there should be a module for Look and Feel themes which should include the default Breeze and Breeze Dark Look and Feel themes. The Look and Feel themes will update all of the relevant themes to the appropriate settings. You might start by selecting and applying the Breeze Dark Look and Feel theme, and then go through the other settings modules to customize specific items afterwards (most applications should automatically update to the new theme, but some applications may need to be restarted).

airdrik wrote:What version of KDE and linux distro are you using?

I assume that you've used KDE's System Settings to change the applications themes to dark (for the Application Theme (for KDE/QT apps), Desktop Theme, Color Scheme and GTK Theme).
In KDE's System Settings, there should be a module for Look and Feel themes which should include the default Breeze and Breeze Dark Look and Feel themes. The Look and Feel themes will update all of the relevant themes to the appropriate settings. You might start by selecting and applying the Breeze Dark Look and Feel theme, and then go through the other settings modules to customize specific items afterwards (most applications should automatically update to the new theme, but some applications may need to be restarted).

Thank you for the answer.
Distro is Linux Mint 19 Tara
KDE Plasma 5.12.6
KDE Frameworks 5.47.0
QT Version 5.9.5

Look and feel already set to Breeze Dark in workspace theme/Look and Feel
Is there anything else I can do?

Looks like the apps in question are running as root with sudo or similar?
There have been a few discussions for getting the theme right for apps run as root, which should prove helpful, including:
viewtopic.php?f=225&t=140091&p=375527#p375527
viewtopic.php?f=289&t=125255&p=374873#p374873