diff -uprN pnmixer-0.7.2-orig/data/ui/prefs-dialog-gtk3.glade pnmixer-0.7.2/data/ui/prefs-dialog-gtk3.glade
--- pnmixer-0.7.2-orig/data/ui/prefs-dialog-gtk3.glade	2017-06-02 16:55:02.000000000 +0200
+++ pnmixer-0.7.2/data/ui/prefs-dialog-gtk3.glade	2018-02-14 15:15:02.626477919 +0100
@@ -1259,6 +1259,9 @@
                 <property name="tab_fill">False</property>
               </packing>
             </child>
+            <style>
+              <class name="frame"/>
+            </style>
           </object>
           <packing>
             <property name="expand">True</property>