[RFE] dark theme

[illume]

Current situation

We have a dark theme, and code for controlling it, but no dark theme has been designed and tested.

Additionally, we don't have a user UI for selecting a dark theme.

Impact

This is a usability / a11y issue.

Ideal future situation

Nebraska detects the users theme preference and uses that, and also allows the user to change it with a UI.

Additional information

The code for theme selection has been left in there, but the Browser/OS detection part has been disabled for now until the dark theme gets more attention. The code follows a similar structure to Headlamp and was added in #509