UPDATE: There was an error in one of the RGB values from the comment above!
As ideal default solid background colours:
For Ambiance - Ubuntu Dark Purple - HEX: #2C001E RGB: 44 0 30 For Radiance - Ubuntu Grey - HEX: #AEA79f RGB: 173 167 159
If we can't define separate colours for Ambiance/Radiance I suggest using the Ambiance colour for both: Ubuntu Dark Purple - HEX: #2C001E RGB: 44 0 30
UPDATE: There was an error in one of the RGB values from the comment above!
As ideal default solid background colours:
For Ambiance - Ubuntu Dark Purple - HEX: #2C001E RGB: 44 0 30
For Radiance - Ubuntu Grey - HEX: #AEA79f RGB: 173 167 159
If we can't define separate colours for Ambiance/Radiance I suggest using the Ambiance colour for both:
Ubuntu Dark Purple - HEX: #2C001E RGB: 44 0 30