Reply To: CSS Layouts – Fixed Sidebar with Background that extends to edge of window

[Beverleyh]

Negative margin pulls the container boundary way past its original position.
Padding puts internal content back into its original position.
It’s a way of extending background colours to the edge of the screen – one of the original ways of achieving seemingly equal height columns (or in this case, width) with CSS before you could fake it with pseudo elements, or do it ‘properly’ with flexbox.