Add System Option for Dark Mode

[0xTim]

Currently we start system mode and then the switcher remembers the preference. We should change this to a drop down menu that allows you to pick between light, dark and system

[0xTim]

We also need to account for the system setting changing - see https://stackoverflow.com/questions/59621784/how-to-detect-prefers-color-scheme-change-in-javascript