## Context

The current UI defaults to dark presentation and lacks a global theme control. Users with different preference and accessibility needs cannot choose light/system/high-contrast alternatives.

This change introduces an icon-based theme switcher and persistent client-side preference restoration.

## Goals / Non-Goals

**Goals:**
- Add a header theme switcher with system, light, dark, and high-contrast options.
- Apply theme choice globally with minimal visual regression.
- Persist preference in localStorage with cookie fallback.
- Restore returning-user choice; default to system when unset.

**Non-Goals:**
- Server-side profile theme persistence.
- Theme-specific content changes.
- Full design-system rewrite.

## Decisions

### Decision: Centralize theme state on `document.documentElement`
**Decision:** Set a root theme attribute/class and drive color tokens from CSS variables.

**Rationale:**
- Single source of truth for whole page styling.
- Works with existing Tailwind utility classes via custom CSS variable bridge.

**Alternatives considered:**
- Component-level theming flags: rejected due to drift and maintenance overhead.

### Decision: Keep system mode dynamic via `prefers-color-scheme`
**Decision:** For `system`, listen to media query changes and update resolved theme automatically.

**Rationale:**
- Matches user OS preference behavior.

**Alternatives considered:**
- One-time system snapshot: rejected as surprising for users changing OS theme at runtime.

### Decision: Use icon-only options with accessible labels
**Decision:** Theme controls are icon buttons with ARIA labels and visible selected state.

**Rationale:**
- Meets UX requirement while preserving accessibility.

**Alternatives considered:**
- Text dropdown: rejected due to explicit icon requirement.

## Risks / Trade-offs

- **[Risk] Existing hardcoded color classes may not adapt perfectly** -> Mitigation: prioritize core surfaces/text and progressively map remaining variants.
- **[Risk] High-contrast mode may expose layout artifacts** -> Mitigation: audit focus outlines, borders, and semantic contrast first.
- **[Trade-off] Additional JS for persistence and media listeners** -> Mitigation: keep logic modular and lightweight.

## Migration Plan

1. Add theme tokens and root theme resolver.
2. Implement icon switcher in header and state persistence.
3. Wire system preference listener and fallback behavior.
4. Validate across refresh/returning sessions and responsive breakpoints.

Rollback:
- Remove theme switcher and resolver, revert to existing dark-default classes.

## Open Questions

- Should high-contrast mode align with OS `prefers-contrast` in a later phase?
