Add dark mode support

[Lakes41]

The README mentions optimisation for light and dark environments, but the Expo config currently uses a light interface style. Add full dark mode support across the app.

Suggested tasks

• Update the app configuration to support system theme
• Add dark mode colour tokens
• Review core screens for dark mode readability
• Test onboarding, profile, guilds, access check, and settings screens
• Fix contrast issues where needed

Acceptance criteria

• The app supports light and dark modes
• Text and backgrounds remain readable in both modes
• Main screens are tested in dark mode

[wavyboy-build]

Dear maintainer please assign me this issue to resolve

[grantfox-oss]

🦊 GrantFox — @wavyboy-build has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #10)
2. Your PR will be reviewed by the Adamantine-guild maintainers

Good luck! Track your progress on GrantFox.