Hey there, I am the customer. 
If I go to _Options / Appearance / Colors and Press [Reset defaults] (German: Standardwerte wiederherstellen), the colors switch back to the original default colors (see 2) until I close the options dialog with either “OK” or “Cancel”. After leaving the options dialog the screen is flickering some microseconds and the ugly dark gray is restored. (see 3).
Rhino 5 SR 12 64-bit 5.12.50810, Win7 Pro 64-bit