"Luka", "Aqua", and "OpenTTD" themes

[WenSimEHRP]

Those would be the default themes for this app.

• Luka
  • Figure out a colour palette (Magenta?)
  • Light
  • Dark
• Aqua
  • Colour palette (Teal?)
  • Light
  • Dark
• OpenTTD
  • Colour palette
  • Light
  • Dark
• Simple, i.e., default egui theme