docs: Write theming and appearance documentation

[ggfevans]

Summary

Document theming, appearance options, and visual customisation in Rackula.

Content Outline

Theme Modes

• Dracula (Dark): Default dark theme with purple accents
• Alucard (Light): Light theme with warm cream tones
• Theme toggle in toolbar
• Persistent theme storage (saved in browser)

System Theme Detection

• Respects system preference on first visit
• Manual override persists across sessions

Colour Palette

• Brand colours (purple, cyan, pink, green)
• Device category colours
• Custom colour assignment per device

Accessibility Considerations

• High contrast mode support
• Reduced motion preferences respected
• WCAG AA compliant colour contrast

Acceptance Criteria

• Both themes documented with screenshots
• Theme toggle location explained
• Colour customisation options documented

Test Requirements

• Build completes without errors
• Screenshots show both themes