This is an easy one.

When I zoom my browser in (with Ctrl+ or Apple+) the left-side navigation margin gets bigger and bigger, pushing the useful text off the screen. That has bothered me (and anyone else with 40+ year old eyes) ever since the switch to the newest doc format with Sphinx.

There is no fix that will satisfy everyone. People with perfect vision might like to zoom out (with Ctrl- or Apple-), since the margin currently gets smaller and smaller. But we need a compromise.

The relevant CSS, in default.css, is this:

 div.bodywrapper {
    margin: 0 0 0 230px;
 }

If instead it were something like this:

 div.bodywrapper {
    margin: 0 0 0 10%;
 }

then at least the navigation margin would stay the same size always.

If you really want it to grow and shrink, then you need some sort of javascript control of its position.