Add a message-dialog style class

This helps in separating message dialogs from other dialogs,
theming-wise.

docs/reference/gtk/gtk3-sections.txt

@@ -5994,6 +5994,7 @@ GTK_STYLE_CLASS_MARK
 GTK_STYLE_CLASS_MENU
 GTK_STYLE_CLASS_MENUBAR
 GTK_STYLE_CLASS_MENUITEM
+GTK_STYLE_CLASS_MESSAGE_DIALOG
 GTK_STYLE_CLASS_NEEDS_ATTENTION
 GTK_STYLE_CLASS_NOTEBOOK
 GTK_STYLE_CLASS_OSD

gtk/gtkstylecontext.h

@@ -807,6 +807,15 @@ struct _GtkStyleContextClass
  */
 #define GTK_STYLE_CLASS_POPUP "popup"
 
+/**
+ * GTK_STYLE_CLASS_MESSAGE_DIALOG:
+ *
+ * A CSS class that is added to message dialogs.
+ *
+ * Since: 3.14
+ */
+#define GTK_STYLE_CLASS_MESSAGE_DIALOG "message-dialog"
+
 /**
  * GTK_STYLE_REGION_ROW:
  *

gtk/resources/ui/gtkmessagedialog.ui

@@ -8,6 +8,9 @@
     <property name="resizable">False</property>
     <property name="type_hint">dialog</property>
     <property name="skip_taskbar_hint">True</property>
+    <style>
+      <class name="message-dialog"/>
+    </style>
     <child internal-child="headerbar">
       <object class="GtkHeaderBar" id="headerbar1">
         <property name="visible">True</property>