diff --git a/src/atm/detector.py b/src/atm/detector.py
index b99e994..f2c3d82 100644
--- a/src/atm/detector.py
+++ b/src/atm/detector.py
@@ -37,13 +37,19 @@ class Detector:
         self,
         cfg: Config,
         capture: ScreenCapture,
-        bg_rgb: tuple[int, int, int] = (18, 18, 18),
-        bg_tol: float = 15.0,
+        bg_rgb: tuple[int, int, int] | None = None,
+        bg_tol: float | None = None,
     ) -> None:
         self._cfg = cfg
         self._capture = capture
-        self._bg_rgb = bg_rgb
-        self._bg_tol = bg_tol
+        # Prefer config-defined background; fall back to dark-grey default.
+        if "background" in cfg.colors:
+            spec = cfg.colors["background"]
+            self._bg_rgb = bg_rgb if bg_rgb is not None else spec.rgb
+            self._bg_tol = bg_tol if bg_tol is not None else spec.tolerance
+        else:
+            self._bg_rgb = bg_rgb if bg_rgb is not None else (18, 18, 18)
+            self._bg_tol = bg_tol if bg_tol is not None else 15.0
         # Palette excludes "background" key
         self._palette: dict[str, tuple[tuple[int, int, int], float]] = {
             name: (spec.rgb, spec.tolerance)