Add design doc for dark/light theme support

Defines the config format, data model, and daemon integration
approach for theme-aware styling.

Made-with: Cursor

docs/plans/2026-03-10-dark-light-theme-design.md

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+# Dark/Light Theme Support — Design
+
+## Summary
+
+Add dark/light mode theme support to cursor-flasher. Users can define separate border styles for each OS appearance mode via new `dark` and `light` config sections. A `theme` option controls which styles are active: `"dark"`, `"light"`, or `"auto"` (follows macOS appearance in real-time).
+
+## Config Format
+
+The old top-level `running`/`completed` format is replaced (breaking change). Modes are now nested under theme sections:
+
+```yaml
+theme: auto  # "dark" | "light" | "auto"
+
+dark:
+  running:
+    color: "#FF9500"
+    width: 4
+    opacity: 0.85
+    pulse_speed: 1.5
+    sound: "Glass"
+    volume: 0.5
+  completed:
+    color: "#00FF00"
+    width: 4
+    opacity: 0.85
+    duration: 1.5
+    sound: ""
+    volume: 0.0
+
+light:
+  running:
+    color: "#3B82F6"
+    width: 4
+    opacity: 0.9
+    pulse_speed: 1.5
+    sound: "Glass"
+    volume: 0.5
+  completed:
+    color: "#22C55E"
+    width: 4
+    opacity: 0.9
+    duration: 1.5
+    sound: ""
+    volume: 0.0
+
+flash:
+  mode: "screen"
+
+approval_tools:
+  - Shell
+  - Write
+  - Delete
+
+general:
+  approval_delay: 2.5
+  cooldown: 2.0
+```
+
+## Data Model
+
+- `StyleConfig` — unchanged (color, width, opacity, duration, pulse_speed, sound, volume).
+- New `ThemeStyles` dataclass — groups `running: StyleConfig` and `completed: StyleConfig` for one theme.
+- `Config` — replaces `running`/`completed` with `dark: ThemeStyles` and `light: ThemeStyles`. Adds `theme: str` field. Exposes `active_styles(system_appearance: str) -> ThemeStyles` method that resolves the correct theme based on the `theme` setting and the passed-in system appearance string.
+
+## Appearance Detection
+
+The daemon detects macOS appearance via `NSApplication.sharedApplication().effectiveAppearance().name()`. If the name contains "Dark", the appearance is `"dark"`; otherwise `"light"`. This check happens at flash/pulse trigger time (not polled), so it picks up OS appearance changes between flashes with zero overhead.
+
+## Daemon Integration
+
+Two call sites change: `_check_pending()` and `_handle_stop()`. Each resolves the active theme styles at trigger time:
+
+```python
+styles = self.config.active_styles(_get_system_appearance())
+self.overlay.pulse(frames, styles.running)
+play_alert(styles.running)
+```
+
+## Decisions
+
+- **Modes under themes** (not themes under modes) — `dark.running` rather than `running.dark`.
+- **Old format not supported** — top-level `running`/`completed` keys are ignored.
+- **Real-time detection** — appearance checked at each flash trigger, not just at startup.
+- **Config stays pure** — no Cocoa imports in config.py; appearance detection lives in daemon.py.