New Instructions that use Arduino built-in theme mechanism.
Download
https://github.com/taunoe/arduino-dark-theme/raw/master/tauno-dark-theme.zip
Find your Arduino Sketchbook location: Arduino IDE > File > Prefernces
In my case /home/tauno/Arduino
Go to that location and make new foler theme
In my case /home/tauno/Arduino/theme
Copy downloaded tauno-dark-theme.zip to theme folder.
Open Arduino IDE > File > Prefernces and select new theme:
Restart Arduino IDE and have fun.
Old Instructions
Tested on Ubuntu 18.04, 18.10 and Windows 10. I do not guarantee that it will work on older OSs.
Download from GitHub: https://github.com/taunoe/arduino-dark-theme
Find your Arduino theme folder.
On Ubuntu its is usualy on Your_Arduino_folder/lib/theme
Like: /home/you/downloads/arduino-1.8.9/lib/theme
On Windows: C:\Program Files (x86)\Arduino\lib\theme
Rename theme folder to theme_old and copy-past the new theme folder.
Restart Arduino IDE and have fun.
Tauno Erik 
Thanks for this. Is it possible to also edit the Serial Monitor screen. Its still bright white?
Yes on Windows 10 it’s white. Unfortunately, I have no idea how to change its colour.