Add alternate code highlighting themes + a switcher

[AmeliaBR]

Solarized Dark fails WCAG AA color contrast.

Probably want four options:

• Solarized Light
• Solarized Dark
• a high-contrast light
• a high-contrast dark

Need to:

• Add CSS files for alternate color schemes (need to remember where I found the Pygments-compatible CSS file)
• Write a short JS utility to
  • inject a select drop-down with the stylesheet options (& maybe style each option so it gives a preview of the colors)
  • save the user's selection choice in local storage
  • check for local storage settings on page load
  • swap the URL on the relevant stylesheet <link> if a stored setting is detected on load, or if the selected option changes
• Add the JS to every page that uses syntax-highlighted code (and regret, once again, that I didn't spend the time to create a proper build & template system)

[AmeliaBR]

Will Boyd suggests an alternative approach, using filters to tweak the color scheme without downloading separate CSS: https://codersblock.com/blog/customizing-github-gists/

[AmeliaBR]

Nice dark colour theme here by Sarah Drasner, but I'd need to convert it to the Pygments class names:
https://twitter.com/sarah_edo/status/997238414390022144