/indra/newview/skins/default/xui/en/widgets/panel.xml

<?xml version="1.0" encoding="utf-8" standalone="yes" ?>
<!-- Optional parameters:
  border - show border around panel
  bg_opaque_image - image name for "in-front" panel look
  bg_alpha_image - image name for "in-back" or transparent panel look
-->
<panel name="panel"
       bg_opaque_color="PanelFocusBackgroundColor"
       bg_alpha_color="PanelDefaultBackgroundColor"
       bg_opaque_image_overlay="White"
       bg_alpha_image_overlay="White"
       background_visible="false"
       background_opaque="false"
       chrome="false"
       accepts_badge="true"/>