Add site-wide toggle for dark mode

[WillieCubed]

Right now, some components respond to the prefers-color-scheme CSS media query based on the browser/system selected theme, but many users would prefer to willingly opt into a dark theme on a site-specific basis. Implementing a toggle for dark theme would require storing some minimal data on the client (a key-value pair indicating theme preference) using Local Storage, but it certainly won't harm functionality.

[WillieCubed]

For updating the favicon, we could use the method described here.