Add dark mode

[felipecrs]

Dark mode was removed with the introduction of the new Web UI.

This brings it back, with a few improvements.

• Added a toggle button to switch themes between Auto, Light, and Dark modes.
• The Auto mode respects the system theme preference. It's the default mode.
• The Network Graph page required some minor adjustments to look nice in Dark mode.

This PR also:

• Upgraded vis-network from v9.1.9 to v10.0.2.

chrome_5OdKYpPWht.mp4

Important

The next demo is outdated (config editor and top bar button), but the rest is valid.

dark.mode.mp4