In your case $this
is an instance of Mage_Page_Block_Html_Head
. You will find the method getCssJsHtml
in that class.
This method generates the html for the css and js files added to the <head>
tag.
In order to add a js file to the head tag you can add this to one of the layout files
<page_handle_here>
<reference name="head">
<action method="addJs"><file>path/to/js/relative/to/js/folder.js</file></action>
</reference>
</page_handle_here>
to add a css file:
<page_handle_here>
<reference name="head">
<action method="addCss"><file>path/to/js/relative/theme/folder.css</file></action>
</reference>
</page_handle_here>