Description:

Running 'gitk' fails with:
application-specific initialization failed: unknown color name "BACKGROUND".

The reason is pollution of (the output of) `xrdb -query`
by files in /usr/share/kdisplay/app-defaults/* which are
part of package extra/plasma-desktop

Neither 'BACKGROUND', 'FOREGROUND', 'WINDOW_BACKGROUND' or 'WINDOW_FOREGROUND'
are valid color names.

Additional info:
* package version(s): plasma-desktop 5.18.5-1
* config and/or log files etc.
* link to upstream bug report, if any

Steps to reproduce:

Install

>pacman -Qe
amd-ucode 20200421.78c0348-1
base 2-2
chromium 81.0.4044.138-1
crda 4.14-3
discord 0.0.10-3
efibootmgr 17-1
git 2.26.2-1
gnome-themes-extra 3.28-1
gnu-free-fonts 20120503-7
gpm 1.20.7.r27.g1fd1941-2
grub 2:2.04-6
hexchat 2.14.3-1
kdialog 20.04.0-1
konsole 20.04.0-1
linux 5.6.11.arch1-1
linux-firmware 20200421.78c0348-1
lvm2 2.02.187-2
man-db 2.9.1-2
man-pages 5.06-2
neovim 0.4.3-3
nvidia 440.82-12
nvidia-settings 440.82-1
openssh 8.2p1-3
pepper-flash 32.0.0.363-1
plasma-desktop 5.18.5-1
rsync 3.1.3-3
schroot 1.6.10-24
strace 5.6-1
sudo 1.8.31.p1-1
synergy 1.10.3-1
telegram-desktop 2.1.2-1
texinfo 6.7-3
tk 8.6.10-1
ttf-dejavu 2.37-3
which 2.21-5
xdg-user-dirs 0.17-2
xorg-fonts-100dpi 1.0.3-4
xorg-fonts-75dpi 1.0.3-4
xorg-fonts-misc 1.0.3-6
xorg-fonts-type1 7.7-3
xorg-server 1.20.8-2
xorg-twm 1.0.10-2
xorg-xclock 1.0.9-1
xorg-xinit 1.4.1-1
xterm 356-1

>cat ~/.xinitrc
exec startplasma-x11

(I have no sddm, not sure if that matters).
I also have no .Xresources etc.

Login and run 'startx'.
Open a konsole and run 'gitk'.