Dark mode toggle in the right-hand sidebar

[ddelange]

Is your feature request related to a problem? Please describe.
Google Chrome (Mac OS X) respects system wide dark mode setting. However, the tab bar becomes barely readable for me when Chrome is in dark mode, so I disabled dark mode for Chrome.

Quoting furo docs on dark mode, it seems I won't be able do view dark furo projects without putting my whole browser in dark mode (unwanted):

Furo is in light mode by default, switching to the dark mode when requested by the user’s browser (through prefers-color-scheme: dark).

As a consequence of this design, the dark mode inherits the variable definitions from the light mode, only overriding specific values to adapt the theme. While the mechanism for switching between light/dark mode is not configurable, the exact CSS variable definitions used in this process can be configured.

Describe the solution you'd like
A nice solution would be to keep this behaviour, but to additionally give the user the option to override the CSS prop with a toggle, like for instance Docker docs have on every page.

Describe alternatives you've considered
Alternatively, I could somehow attempt hack my Chrome set up to emulate this CSS prop on all websites by default, essentially putting my Chrome in dark mode only for the content and not for Chrome's own UI.

Additional context
Docker docs example dark mode toggle on the right:

[pradyunsg]

Same as #24. :)