Add Dark mode to Nomad UI

Hi there,
I have not found any information regarding a dark mode for the Nomad UI, so I wanted to know if it would be of any interest for the UI team to implement one.

### Proposal
Add controls on the UI to allow toggling between dark, light and system modes

### Use-cases
Users can set their preferred theme at will according to their setup

### Attempted Solutions
There are third party browser extension that can change the theme of web pages but wont always change the right elements to the right colour and styling. It would be really nice to have a simple dark mode from the team itself.

Start by mapping the Nomad UI entry points and any existing theme or appearance handling relevant to the proposed controls. Done means users can switch reliably among dark, light, and system modes across the UI; the issue names no specific files or tests, so the implementation scope needs investigation first.

Written by the indexing model from the issue text.