Hi @friech,
I tried pasting your code inside:
Customize > Custom > Edit Global CSS
but nothing happened, then I removed the code and tried pasting it into:
Customize > Custom > Additional CSS - but again, nothing happened.
As you can see from these two images:
I tried adding #eaeaea color, and red color #ff0000 as you suggested, but again, nothing really changes.
Anyway @friech I think this is not the solution:
As you can see, I have another X website ( http://bit.ly/2woJg9Q ) with a dark grey footer
and the error is there as well.
The sidebar always crashes / disappears inside the footer,
even if I added the code @paul.r suggested:
.q2w3-fixed-widget-container { z-index: 9999;}