don't pick up system dark theme in -alternate Mac build

The older Qt doesn't support it properly

qt/aqt/theme.py

@@ -9,7 +9,8 @@ from typing import Dict
 from anki.utils import isMac
 from aqt import QApplication, gui_hooks, isWin
 from aqt.colors import colors
-from aqt.qt import QColor, QIcon, QPalette, QPixmap, QStyleFactory, Qt
+from aqt.qt import QColor, QIcon, QPalette, QPixmap, QStyleFactory, Qt, \
+    qtminor
 
 
 class ThemeManager:
@@ -23,6 +24,8 @@ class ThemeManager:
             return False
         if not isMac:
             return False
+        if qtminor < 14:
+            return False
         import darkdetect  # pylint: disable=import-error
 
         return darkdetect.isDark() is True