Remove horizontal padding in region-main

[Info 3bits]

Remove horizontal padding in region-main. Actually 10px

[Alexander Goryntsev]

Pls. consider also layouts with different colors for the site background and for the region-main. How it will look without region-main padding?

[Info 3bits]

Apply a background colour to the region-main is not nice because it is applied only to the margin between elements.

To get a better result, it should be applied also to the sidebar but it is not looking well too.

The horizontal padding was removed in a previous commit.

I will open another issue to remove the container in the Marketing Blocks region that actually has a padding.