preference-sensing persistent dark mode toggle

[burkfers]

Both light and dark stylesheets are loaded.
If the local storage item is present, that theme is applied.
If it is not present, one style is applied based on browser-supplied preference by means of css @media and light or dark is saved in local storage.
The button toggles the style immediately and saves the preference in local storage, overriding the browser-supplied preference.

[burkfers]

The callout colors need to be adjusted for the dark theme. I've made them explicit for your tweaking pleasure, see assets/css/just-the-docs-bkb-dark.scss.

[burkfers]

A little prettier: The color scheme toggle and the github aux link are now images.

[bstiq]

Great work here.
As far as we can see visually everything looks good.
Let me know if you're also happy with the current code. If no more background work is needed I'll merge the PR.

[burkfers]

Only the issue that the toggle is not shown in mobile view remains, which I've broken out into #3.

Since we also respect the preference supplied by the browser, the selected view should be correct most of the time. Imho, the benefits of merging this now outweigh the drawback of possibly locking mobile users into a theme they didn't want.