My Arduino dark theme. I changed UI colors, icons and syntax color.
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
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.