Make dark mode work well

[davidfarmer]

Add known issues with using the browser's built-in dark mode.

Please include a link to the HTML and which browser had an issue.

[oscarlevin]

Using Chrome's dark mode doesn't do anything for my book at all. There is a "Dark" extension that forces sites to use a dark mode, and this does invert the colors. However, it doesn't invert the colors of SVG files, so they look like black dots on very dark gray. https://discrete.openmathbooks.org/dmoi3/sec_gt-intro.html

[ascholerChemeketa]

This PR is the one that turns on the existing dark css rules when dark mode is set by the OS/browser.

#2114

[rbeezer]

Sample article, with #2114 applied, is at https://pretextbook.org/beta/html-dark-2024-01-26

[davidfarmer]

In the sample article:
https://pretextbook.org/beta/html-dark-2024-01-26/section-interesting-corollary.html

Checkpoint 4.4 knowl has white background (needs to be dark, not necessarily the
same as the page background).

Same for Checkpoint 4.7 (also need to address the H/A/S in that knowl)

Solution in Exploration 4.2 knowl looks fine (but maybe use a different dark background?)

Aside near Checkpoint 4.10 needs dark background (but definitely different than the
main dark background).

Sage cell in Technology 4.12 has stark white background. Make less white, or should
Sage cells be light text on dark background?

4.2.6 Solutions for This Subsection:
white border on left should be less light

[rbeezer]

Lots to chew on in the "Graphics" section. LaTeX-images (tikz) are likely based on an old conversion from PDF to SVG, it is possible they might behave differently if regenerated.

https://pretextbook.org/beta/html-dark-2024-01-26/section-graphics.html

Asymptote images all seem to have a solid-white background. But transparent images elsewhere don't seem to work so well, things like axes are being washed out. Not clear to me what we want here, and if we will need to advise authors on constructing their images.