emacs does not start, the error is the following:
Undefined color: "WINDOW_FOREGROUND"

According to emacs dev (?), this is outside of Emacs control. There are some files that expects to be run through the preprocessor, but isn't. (sic)
http://emacs.1067599.n8.nabble.com/bug-16462-emacs-does-not-start-under-KDE-Undefined-color-quot-WINDOW-FOREGROUND-quot-td310437.html

It seems that this happens because KDE's defaults apply its color and font definitions even to applications that weren't compiled for KDE. Interesting link (from line 1203): https://fossies.org/linux/emacs/etc/PROBLEMS

I try to remove /usr/share/kdisplay/app-defaults/Emacs.ad or make this file empty. In these cases, the error is now the following:
Undefined color: "FOREGROUND"

I want to keep the breeze dark style for my GTK applications (System Settings > Application Style > GNOME Application Style (GTK)) so I didn't uncheck anything in those GTK settings.

STEPS TO REPRODUCE
1. 
2. 
3. 

OBSERVED RESULT
Undefined color: "WINDOW_FOREGROUND"

EXPECTED RESULT


SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: Arch Linux 
(available in About System)
KDE Plasma Version: 5.17.2
KDE Frameworks Version: 5.64.0
Qt Version: 5.13.2

ADDITIONAL INFORMATION
It is my first bug report (ever), I searched but I didn't find any bug report of this kind in the KDE Bug Report.