Problem solved, thanks to ProcessMonitor (from Sysinternals) and RegEdit. 
I removed the following registry entry that was set to 13160660 = 0xC8D0D4 = 200,208,212 BRG = 212,208,200 RGB = the dark gray in my upper screen shot and everything is back to normal now:
HKEY_CURRENT_USER\Software\McNeel\Rhinoceros\5.0x64\Scheme: Default\Options\UiPaintColors\tab_background