Which controls?

The style property for that is called sectionmain

The best way to find that is to go into your Admin CP, and search in the top right corner for sectionmain

One of the results will be a style property.

There you can change the background colour and the border.
 
Another route if you only wanted to style that area without it affecting the forum_list or other areas.which would be added in EXTRA

Code:
.discussionList.sectionMain  {
    background-color: red !important;
    border: 1px solid darkred !important;
}
 
Without EXTRA.css, styling XF would be very difficult. When a styling property applies to all iterations of an item, EXTRA is the only way to apply it to one iteration only. In one instance I found it easier to let styling properties apply to all, then I used EXTRA to remove the styling from the one area I didn't want styled.
 
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