Themes: Better visibility of scrollbar

gajim/data/gui/themes_window.ui

@@ -10,7 +10,8 @@
         <property name="can_focus">True</property>
         <property name="hscrollbar_policy">never</property>
         <property name="shadow_type">in</property>
-        <property name="min_content_height">200</property>
+        <property name="min_content_height">260</property>
+        <property name="overlay_scrolling">False</property>
         <child>
           <object class="GtkViewport">
             <property name="visible">True</property>