Make Skype's tray icon monochrome on Linux
TL;DR: Use skype1ico
This was done on Arch Linux and KDE Plasma 5, but other systems will work
Remember the path to this skype1ico folder.
Make this shell script which you will start Skype with:
And here's the theme itself (paste this into the file):
This theme just uses the environment's native icons, which are hopefully monochrome or better fitting.
The supplied picture-based themes don't seem to work with KDE Plasma...