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.