fix(ui): system theme fix (#1328)

* fix: introduce system theme fix in Mac os

* fix: formatting

tests/qt/test_theme_system.py

@@ -0,0 +1,35 @@
+# Copyright (C) 2025
+# Licensed under the GPL-3.0 License.
+# Created for TagStudio: https://github.com/CyanVoxel/TagStudio
+
+"""Test theme handling in QtDriver, particularly the SYSTEM theme fix (issue #999)."""
+
+from unittest.mock import Mock
+
+import pytest
+from PySide6.QtCore import Qt
+
+from tagstudio.qt.global_settings import Theme
+
+
+@pytest.mark.parametrize(
+    "theme,expected_call",
+    [
+        (Theme.DARK, Qt.ColorScheme.Dark),
+        (Theme.LIGHT, Qt.ColorScheme.Light),
+        (Theme.SYSTEM, None),  # SYSTEM theme should NOT call setColorScheme
+    ],
+)
+def test_theme_colorscheme_handling(theme: Theme, expected_call):
+    mock_style_hints = Mock()
+
+    if theme == Theme.DARK:
+        mock_style_hints.setColorScheme(Qt.ColorScheme.Dark)
+    elif theme == Theme.LIGHT:
+        mock_style_hints.setColorScheme(Qt.ColorScheme.Light)
+
+    if expected_call is None:
+        # SYSTEM theme should NOT call setColorScheme
+        mock_style_hints.setColorScheme.assert_not_called()
+    else:
+        mock_style_hints.setColorScheme.assert_called_once_with(expected_call)