Support dark mode on the admin interface

[Kallb123]

Previously only half the interface would "support" dark mode (some elements would go dark and the text would be unreadable). Now, an optional dark mode has been applied across the admin interface, which is enabled based upon your system setting. This is how it was already half-implemented by the component library.

Before:

After:

How to Test

1. bun dev
2. Open the admin interface http://localhost:5173/#/
3. Navigate through pages and see the standard light theme.
4. Change your OS theme to dark. This seems to be different to the browser setting (which doesn't change websites, just the browser UI like tabs and menus).
5. The admin interface should update with a dark theme when the OS theme changes. Navigate through the pages to evaluate the dark mode.