I’m tired of confusing myself trying to write up a description of the layout. Here it is, make of it whatever you will.
The primary design idea was two columns where the widths of the columns had a maximum width, but the background color for each column would extend beyond the content to the edges of the viewport while the content, as a whole, remains centered to the viewport.
I took a few different approaches and eventually landed on using media queries to apply different approaches to the same problem depending on the situation with the viewport (are we at max-width yet, or not?) It works. I perhaps made it more complicated than most people really need it to be, but you can pick out the bits you don’t need to keep things simpler.
The CSS is heavily commented with variables and formulas to help keep everything pixel-perfect. It’s a layout like this one that makes me wish CSS had some kind of built-in support for variables. It’d make creating these kinds of layouts a lot easier.