Color-mode / dark-mode support

[chalin]

• Contributes to Support dark mode #331
• The Bootstrap docs invite us to copy their color-theme picker code. Well, I did just that :)
• Adds a color theme picker partial
• Illustrates the use of `` in the UG, which causes the color-theme picker to be added to the end of the navbar, just before the search box
• I'll create a changelog entry in a separate PR. My intent is to make clear that this early support is experimental in alpha, and still subject to change.

Preview: https://deploy-preview-1909--docsydocs.netlify.app/docs/

Screenshots

[chalin]

I just noticed a few cases I missed -- for a growing list of elements to fix (usually by adjusting the background color), see the opening comment of #331. Marking this as draft in the meantime.

[chalin]

Changed my mind: let's get this in and I'll submit separate followup PRs.