- fixes #6102

- the point is that we don't set any style for the group boxes, therefore the Windows style is applied as nothing was set yet. The default Windows style is the color #dddddd which is hardly visible on our blue default style.
as solution set an explicit color.

src/Gui/QSint/actionpanel/actionpanelscheme.cpp

@@ -89,6 +89,22 @@ const char* ActionPanelDefaultStyle =
         "background-color: #ddeeff;"
         "color: #006600;"
     "}"
+
+    // set a QGroupBox  to avoid that the OS styles it, see
+    // https://github.com/FreeCAD/FreeCAD/issues/6102
+    // the px values are taken from Behave-dark.qss, except the padding
+    "QSint--ActionGroup QFrame[class='content'] QGroupBox {"
+    "border: 1px solid #bbbbbb;"
+    "border-radius: 3px;"
+    "margin-top: 10px;"
+    "padding: 2px;"
+    "}"
+    // since we set a custom frame we also need to set the title
+    "QSint--ActionGroup QFrame[class='content'] QGroupBox::title {"
+    "padding-left: 3px;"
+    "top: -6px;"
+    "left: 12px;"
+    "}"
 ;