Add a dark mode

[grafana-dee]

Add a dark mode and read the device settings to enable it.

Stretch goal: Add a profile option so that someone could define a universal override of dark/light based on their preference rather than their device.

[grafana-dee]

Note that at the moment the CSS is built from a very old version of Bootstrap that may no longer be buildable: https://github.com/microcosm-cc/microweb-bootstrap