Enable dark theme

[Marc-Andrieu]

Create a dark mode version for the client.

1. Have a dark theme that works: currently, most colors are hardcoded

• So the UI will hardly see any changes if we say "the theme is dark"; it lacks the flexibility of some "color factorization".
• Therefore we'll have to agree on some sets of colors.

2. Decide what chooses the theme:

• What is the default? Light theme or system theme?
• Allow users to toggle the theme by means of a switch in the UI
• Remember the user's preference. If no, it might bother the user... If yes:
  • Remember w.r.t. the user?
    • I don't see any advantage to it: involving Hyperion (and to store a UI-related thing); the correct theme would only be applied after logging in + the usual loading time; and what if a user wants the dark theme on phone but the light theme on computer?
  • Remember device-wise?
    • Definitively easier to implement (Hyperion not involved), better UX (the login page could be dark!), more flexible for multiple devices.