Better theme switcher with dropdown

[ericswpark]

Is your feature request related to a problem? Please describe.
The current theme switcher button just switches between light mode and dark mode, and saves the preference semi-permanently in the user's local storage. There's no way to get the website to follow the system theme.

Describe the solution you'd like
Change the theme switcher button into a theme switcher "dropdown" with the following options:

• Light
• Dark
• Follow system theme

Describe alternatives you've considered
Three-stage theme switcher button alternating between light, dark, and "follow system theme" (maybe current icon with (A) added in front to indicate "automatic" mode?)

Actually after writing out that alternative that sounds pretty cool too :D