diff --git a/tests/test_tray.py b/tests/test_tray.py
index 3604278..c29410c 100644
--- a/tests/test_tray.py
+++ b/tests/test_tray.py
@@ -57,3 +57,51 @@ class TestNoOpTray:
         from whisper_local.tray._tray import AppState, NoOpTray
         tray = NoOpTray()
         tray.set_state(AppState.RECORDING)  # kein Fehler
+
+
+@pytest.mark.skipif(sys.platform != "win32", reason="Theme nur auf Windows")
+class TestApplySystemTheme:
+    def test_applies_light_theme(self):
+        import tkinter as tk
+        from unittest.mock import patch, MagicMock
+        from whisper_local.tray._theme import apply_system_theme
+
+        root = tk.Tk()
+        root.withdraw()
+        try:
+            with patch("darkdetect.theme", return_value="Light"), \
+                 patch("sv_ttk.set_theme") as mock_set:
+                apply_system_theme(root)
+                mock_set.assert_called_once_with("light")
+        finally:
+            root.destroy()
+
+    def test_applies_dark_theme(self):
+        import tkinter as tk
+        from unittest.mock import patch
+        from whisper_local.tray._theme import apply_system_theme
+
+        root = tk.Tk()
+        root.withdraw()
+        try:
+            with patch("darkdetect.theme", return_value="Dark"), \
+                 patch("sv_ttk.set_theme") as mock_set:
+                apply_system_theme(root)
+                mock_set.assert_called_once_with("dark")
+        finally:
+            root.destroy()
+
+    def test_falls_back_to_light_when_none(self):
+        import tkinter as tk
+        from unittest.mock import patch
+        from whisper_local.tray._theme import apply_system_theme
+
+        root = tk.Tk()
+        root.withdraw()
+        try:
+            with patch("darkdetect.theme", return_value=None), \
+                 patch("sv_ttk.set_theme") as mock_set:
+                apply_system_theme(root)
+                mock_set.assert_called_once_with("light")
+        finally:
+            root.destroy()
diff --git a/whisper_local/tray/_theme.py b/whisper_local/tray/_theme.py
new file mode 100644
index 0000000..ecbcf68
--- /dev/null
+++ b/whisper_local/tray/_theme.py
@@ -0,0 +1,12 @@
+"""System-Theme-Erkennung und sv-ttk-Anwendung."""
+
+import tkinter as tk
+
+
+def apply_system_theme(root: tk.Tk) -> None:
+    """Setzt sv-ttk-Theme passend zum Windows-System-Theme (Light/Dark)."""
+    import darkdetect
+    import sv_ttk
+
+    detected = darkdetect.theme() or "Light"
+    sv_ttk.set_theme(detected.lower())