[CSS] Increase dark mode contrast to meet WCAG requirements and other dark mode things

[sneakers-the-rat]

Main Stuff

The light mode of the packaging guide has a lovely, consistent, white and purple theme. The dark mode leaves a lot of default variables in place, and importantly doesn't switch the H1 color which is a very dark purple (which looks lovely in light mode!) This PR nudges us towards a more consistent style across light and dark modes by extracting some of the colors used elsewhere in the custom css file and creating a set of pyos css variables to use as a palette, and then uses those in the dark mode!

Currently the page looks like this:

which is not great for accessibility since the contrast is so low, and the teal color is pretty grating against the purples. The contrast is beneath WCAG

So the after shot is like this:

which is not perfect but is a step in the right direction.

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Other Stuff

Navbar

There's a big pointless block beneath the navbar stuff, not sure what it it's supposed to do, but at the moment what it does is make a scrollbar always appear and makes the readthedocs version box slide off the page. So I set that things margin to "unset"

Before:

after:

Nox Build

The sphinx-autobuild command doesn't copy static files unless you tell it to (same as sphinx), which was super annoying to work on the CSS with, so i added a section there to allow us to explicitly specify files, and then did that (which is overridable by passing positional arguments)

[lwasser]

AMAZING!!! THANK YOU THANK YOU THANK YOU!!

[lwasser]

pre-commit.ci autofix

[lwasser]

The sphinx-autobuild command doesn't copy static files unless you tell it to (same as sphinx), which was super annoying to work on the CSS with, so i added a section there to allow us to explicitly specify files, and then did that (which is overridable by passing positional arguments)

this has been driving me crazy. the only way to trigger a css update is to make a random edit to another md file and then it updates. thanks for fixing this all. i'll merge this now!