GUI_RC is an XML file which describes what the SHIVA frame should look like. It is written in CookSwing. shiva.ShivaGUI class extends JFrame to have an setter for skinlf for loading SkinLF related themepack.

The default SHIVA GUI frame looks like:

<frame varref="this" title="SHIVA" size="640,480" iconimage="shiva/resources/icons/shiva_icon.gif" defaultcloseoperation="DO_NOTHING_ON_CLOSE" windowlistener="windowCloseListener">
   <uimanager systemlookandfeel="true"/>
</frame>

MBAT changes it a bit to load its own theme pack.

<frame varref="this" title="MBAT" size="640,480" iconimage="trogdor/resources/icons/mouseBirnFavicon.gif" defaultcloseoperation="DO_NOTHING_ON_CLOSE" windowlistener="windowCloseListener" skinlf="/trogdor/resources/themepack.zip">
</frame>