diff --git a/tests/mock_helpers.py b/tests/mock_helpers.py
index c7c991487..737039faf 100644
--- a/tests/mock_helpers.py
+++ b/tests/mock_helpers.py
@@ -22,10 +22,11 @@
 
 def create_toolboxui():
     """Returns ToolboxUI, where QSettings among others has been mocked."""
-    with mock.patch("spinetoolbox.plugin_manager.PluginManager.load_installed_plugins"), mock.patch(
-        "spinetoolbox.ui_main.QSettings.value"
-    ) as mock_qsettings_value:
+    with (mock.patch("spinetoolbox.plugin_manager.PluginManager.load_installed_plugins"), mock.patch(
+        "spinetoolbox.ui_main.QSettings.value") as mock_qsettings_value,
+          mock.patch("spinetoolbox.ui_main.ToolboxUI.set_app_style") as mock_set_app_style):
         mock_qsettings_value.side_effect = qsettings_value_side_effect
+        mock_set_app_style.return_value = True
         toolbox = ToolboxUI()
     return toolbox
 
@@ -41,16 +42,18 @@ def create_project(toolbox, project_dir):
 def create_toolboxui_with_project(project_dir):
     """Returns ToolboxUI with a project instance where
     QSettings among others has been mocked."""
-    with mock.patch("spinetoolbox.ui_main.ToolboxUI.save_project"), mock.patch(
+    with (mock.patch("spinetoolbox.ui_main.ToolboxUI.save_project"), mock.patch(
         "spinetoolbox.ui_main.QSettings.value"
-    ) as mock_qsettings_value, mock.patch("spinetoolbox.ui_main.QSettings.setValue"), mock.patch(
+    ) as mock_qsettings_value, mock.patch("spinetoolbox.ui_main.ToolboxUI.set_app_style"
+    ) as mock_set_app_style, mock.patch("spinetoolbox.ui_main.QSettings.setValue"), mock.patch(
         "spinetoolbox.ui_main.QSettings.sync"
     ), mock.patch(
         "spinetoolbox.plugin_manager.PluginManager.load_installed_plugins"
     ), mock.patch(
         "spinetoolbox.ui_main.QScrollArea.setWidget"
-    ):
+    )):
         mock_qsettings_value.side_effect = qsettings_value_side_effect
+        mock_set_app_style.return_value = True
         toolbox = ToolboxUI()
         toolbox.create_project(project_dir)
     return toolbox