feat: add ACTIVE_THEME global and set_active_theme() to config

engine/config.py

@@ -104,3 +104,26 @@ def set_font_selection(font_path=None, font_index=None):
         FONT_PATH = _resolve_font_path(font_path)
     if font_index is not None:
         FONT_INDEX = max(0, int(font_index))
+
+
+# ─── THEME MANAGEMENT ─────────────────────────────────────────
+ACTIVE_THEME = None
+
+
+def set_active_theme(theme_id: str = "green"):
+    """Set the active theme by ID.
+
+    Args:
+        theme_id: Theme identifier ("green", "orange", or "purple")
+                  Defaults to "green"
+
+    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)