feat(theme): Add theme system with ACTIVE_THEME support

- Add Theme class and THEME_REGISTRY to engine/themes.py
- Add set_active_theme() function to config.py
- Add msg_gradient() function to use theme-based message gradients
- Support --theme CLI flag to select theme (green, orange, purple)
- Initialize theme on module load with fallback to default

engine/config.py

@@ -132,6 +132,7 @@ class Config:
     display: str = "pygame"
     websocket: bool = False
     websocket_port: int = 8765
+    theme: str = "green"
 
     @classmethod
     def from_args(cls, argv: list[str] | None = None) -> "Config":
@@ -175,6 +176,7 @@ class Config:
             display=_arg_value("--display", argv) or "terminal",
             websocket="--websocket" in argv,
             websocket_port=_arg_int("--websocket-port", 8765, argv),
+            theme=_arg_value("--theme", argv) or "green",
         )
 
 
@@ -246,6 +248,40 @@ DEMO = "--demo" in sys.argv
 DEMO_EFFECT_DURATION = 5.0  # seconds per effect
 PIPELINE_DEMO = "--pipeline-demo" in sys.argv
 
+# ─── THEME MANAGEMENT ─────────────────────────────────────────
+ACTIVE_THEME = None
+
+
+def set_active_theme(theme_id: str = "green"):
+    """Set the active theme by ID.
+
+    Args:
+        theme_id: Theme identifier from theme registry (e.g., "green", "orange", "purple")
+
+    Raises:
+        KeyError: If theme_id is not in the theme registry
+
+    Side Effects:
+        Sets the ACTIVE_THEME global variable
+    """
+    global ACTIVE_THEME
+    from engine import themes
+
+    ACTIVE_THEME = themes.get_theme(theme_id)
+
+
+# Initialize theme on module load (lazy to avoid circular dependency)
+def _init_theme():
+    theme_id = _arg_value("--theme", sys.argv) or "green"
+    try:
+        set_active_theme(theme_id)
+    except KeyError:
+        pass  # Theme not found, keep None
+
+
+_init_theme()
+
+
 # ─── PIPELINE MODE (new unified architecture) ─────────────
 PIPELINE_MODE = "--pipeline" in sys.argv
 PIPELINE_PRESET = _arg_value("--pipeline-preset", sys.argv) or "demo"
@@ -256,6 +292,9 @@ PRESET = _arg_value("--preset", sys.argv)
 # ─── PIPELINE DIAGRAM ────────────────────────────────────
 PIPELINE_DIAGRAM = "--pipeline-diagram" in sys.argv
 
+# ─── THEME ──────────────────────────────────────────────────
+THEME = _arg_value("--theme", sys.argv) or "green"
+
 
 def set_font_selection(font_path=None, font_index=None):
     """Set runtime primary font selection."""

engine/render/gradient.py

@@ -80,3 +80,57 @@ def lr_gradient_opposite(rows, offset=0.0):
         List of lines with complementary gradient coloring applied
     """
     return lr_gradient(rows, offset, MSG_GRAD_COLS)
+
+
+def msg_gradient(rows, offset):
+    """Apply message (ntfy) gradient using theme complementary colors.
+
+    Returns colored rows using ACTIVE_THEME.message_gradient if available,
+    falling back to default magenta if no theme is set.
+
+    Args:
+        rows: List of text strings to colorize
+        offset: Gradient offset (0.0-1.0) for animation
+
+    Returns:
+        List of rows with ANSI color codes applied
+    """
+    from engine import config
+
+    # Check if theme is set and use it
+    if config.ACTIVE_THEME:
+        cols = _color_codes_to_ansi(config.ACTIVE_THEME.message_gradient)
+    else:
+        # Fallback to default magenta gradient
+        cols = MSG_GRAD_COLS
+
+    return lr_gradient(rows, offset, cols)
+
+
+def _color_codes_to_ansi(color_codes):
+    """Convert a list of 256-color codes to ANSI escape code strings.
+
+    Pattern: first 2 are bold, middle 8 are normal, last 2 are dim.
+
+    Args:
+        color_codes: List of 12 integers (256-color palette codes)
+
+    Returns:
+        List of ANSI escape code strings
+    """
+    if not color_codes or len(color_codes) != 12:
+        # Fallback to default green if invalid
+        return GRAD_COLS
+
+    result = []
+    for i, code in enumerate(color_codes):
+        if i < 2:
+            # Bold for first 2 (bright leading edge)
+            result.append(f"\033[1;38;5;{code}m")
+        elif i < 10:
+            # Normal for middle 8
+            result.append(f"\033[38;5;{code}m")
+        else:
+            # Dim for last 2 (dark trailing edge)
+            result.append(f"\033[2;38;5;{code}m")
+    return result