From 695b3923c987f2a28ace88b2f5672108d6d90313 Mon Sep 17 00:00:00 2001
From: Juozas Skarbalius <20264107+terahidro2003@users.noreply.github.com>
Date: Wed, 29 Apr 2026 01:45:02 +0200
Subject: [PATCH] fix(ui): system theme fix (#1328)

* fix: introduce system theme fix in Mac os

* fix: formatting
---
 src/tagstudio/qt/ts_qt.py     | 12 ++++++------
 tests/qt/test_theme_system.py | 35 +++++++++++++++++++++++++++++++++++
 2 files changed, 41 insertions(+), 6 deletions(-)
 create mode 100644 tests/qt/test_theme_system.py

diff --git a/src/tagstudio/qt/ts_qt.py b/src/tagstudio/qt/ts_qt.py
index ab81b4e2..5a7b2fd6 100644
--- a/src/tagstudio/qt/ts_qt.py
+++ b/src/tagstudio/qt/ts_qt.py
@@ -306,13 +306,13 @@ class QtDriver(DriverMixin, QObject):
             sys.argv += ["-platform", "windows:darkmode=2"]
         self.app = QApplication(sys.argv)
         self.app.setStyle("Fusion")
-        if self.settings.theme == Theme.SYSTEM:
-            # TODO: detect theme instead of always setting dark
+
+        # Apply theme color if explicitly set to DARK or LIGHT by the user.
+        # For SYSTEM, we let Qt decide based on OS theme.
+        if self.settings.theme == Theme.DARK:
             self.app.styleHints().setColorScheme(Qt.ColorScheme.Dark)
-        else:
-            self.app.styleHints().setColorScheme(
-                Qt.ColorScheme.Dark if self.settings.theme == Theme.DARK else Qt.ColorScheme.Light
-            )
+        elif self.settings.theme == Theme.LIGHT:
+            self.app.styleHints().setColorScheme(Qt.ColorScheme.Light)
 
         if (
             platform.system() == "Darwin" or platform.system() == "Windows"
diff --git a/tests/qt/test_theme_system.py b/tests/qt/test_theme_system.py
new file mode 100644
index 00000000..94a0bdac
--- /dev/null
+++ b/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)