Add regions background as a setting

[Info 3bits]

Add a setting to change the background colour of: region-main, region-side-pre and region-side-post

[Alexander Goryntsev]

Good idea; I find it already half realized with a new 'region-main' setting. However after this we have to check all paddings: it could look fine when it is 'white on white', but with a new region color on a contrast background the layout becomes highly visible and may need some further improvements.

I'm eager to test it.