The simple theme head template only does one thing: it prints out an HTML <link/> to the CSS required for the datepicker tag to render properly.

The source of the simple head.ftl template is:

<#if parameters.calendarcss?exists>
<link rel="stylesheet" href="<@ww.url value='/webwork/jscalendar/${parameters.calendarcss?html}' encode='false' includeParams='none'/>" type="text/css"/>
</#if>