Dark/light mode toggle in navbar

[rerpha]

as a user of the web dashboard I would like a dark/light mode toggle in the navigation bar.

By default the web dsahboard should use the system's native them and toggle from there.

acceptance criteria

• Dark mode button is added in the navbar that applies a dark/light theme to all pages
• pages are modified to have dark mode styles

[rerpha]

@Spencer-Cheverton may be a good one if you need some more tickets for your students