How to change the background of the default color theme?

I can't find a way to change the color of the black area at the bottom of the page, that lies below the footer (in my case the footer is disabled). This is a problem, because the area covers the navigation arrows, when the window gets smaller. It seems that this area isn't a block, because it can't be selected in the edit mode, so it must be some kind of "background", but I can't find the appropriate entry in Design > Site Styles > Colors > Customize your themes, because I've already changed all entries there, but with no effect. Only one approach has an effect: selecting another color in Design > Site Styles > Colors > Edit Palette in the far right circle (read, 5th from left), but this changes other colors in my themes as well, so it's not a real solution.

Observed under Squarespace 7.1. This area also appears, if all CSS tweaks are disabled.

Site with PW "knockknock".

Edit: As it turned out, the black stripe was due to the background color of the global default color theme, as mentioned here.

For a solution see my answer below.

Add to Design > Custom CSS

body {
    background-color: white;
}

This works, thanks!

Is there also another way to change the color by using Squarespace's interface? The downside of this CSS method is that you have to adjust the color manually, if you'll use another color theme.