Text taking up the whole screen makes it so much harder to read and process. Honestly even though I can see why you want to integrate the branding with python.org, it kinda sucks that all the UX advantages carefully designed into RTD has to go with it.

This is one of the no-brainer things that the RTD theme gets right. (And, I guess, that python.org still gets horribly wrong)

I'd advocate something like

div.body {
    max-width: 60em;
}

body {
    max-width: 80em;
}

Which looks like