Add dark mode theme for viewing the WebUI

[mat7ias]

Is your feature request related to a problem? Please describe. The WebUI is mostly white, it'd be nice on the eyes to have a 2nd darker theme to switch to.

Describe the solution you'd like A dark theme and toggle switch for the WebUI.

Additional context An example of the switch might be something similar to what's found on IDEX. I toyed around a bit to make a rough dark theme example: