Adds automatic theme switching with optional toggle

[mattxwang]

Draft as I continue working on this. I intend to tidy up and merge this in first-party.

Builds off of #1086, #1099, #560, #858. Closes #234.

[mattxwang]

Note to myself (and for anybody following along):

• the flashes of unstyled content are pretty bad and persistent in-between pages. perhaps I'll need to rethink the broad-stroke approach, but I'm optimistic I can do some fine-tuning to resolve this.

Still left:

• broadly: incorporate the review comments i made in Implement toggle color scheme, automatic theme mode #1086
• resolve (or mitigate) said flashes
• proper licensing of icons
• properly test each combination of options
• fix accessibility issues on button toggles / state
• potentially clean up JS code
• all the documentation