piCal is the oldest module as my work about xoops.
Thus, many features which standard modules have are not implemented yet.
Template system of xoops is one of them.
You need hack piCal.php by yourself.
The place should be hacked is near line 640 of piCal.php
$ret = "
<h2>"._PICAL_MB_TITLE_EVENTINFO." <small>-"._PICAL_MB_SUBTITLE_EVENTDETAIL."-</small></h2>
<table border='0' cellpadding='0' cellspacing='2'>
<tr>
<td class='head'>"._PICAL_TH_SUMMARY."</td>
<td class='even'>$summary</td>
</tr>
<tr>
<td class='head'>"._PICAL_TH_STARTDATETIME."</td>
<td class='even'>$start_datetime_str</td>
</tr>
<tr>
<td class='head'>"._PICAL_TH_ENDDATETIME."</td>
<td class='even'>$end_datetime_str</td>
</tr>
<tr>
<td class='head'>"._PICAL_TH_LOCATION."</td>
<td class='even'>$location</td>
</tr>
<tr>
<td class='head'>"._PICAL_TH_CONTACT."</td>
<td class='even'>$contact</td>
</tr>
<tr>
<td class='head'>"._PICAL_TH_DESCRIPTION."</td>
<td class='even'>$description</td>
</tr>
<tr>
<td class='head'>"._PICAL_TH_CATEGORIES."</td>
<td class='even'>$categories</td>
</tr>
<tr>
<td class='head'>"._PICAL_TH_SUBMITTER."</td>
<td class='even'>$submitter_info</td>
</tr>
<tr>
<td class='head'>"._PICAL_TH_CLASS."</td>
<td class='even'>$class_status</td>
</tr>
<tr>
<td class='head'>"._PICAL_TH_RRULE."</td>
<td class='even'>$rrule</td>
</tr>
".($this->isadmin?"<tr>
<td class='head'>"._PICAL_TH_ADMISSIONSTATUS."</td>
<td class='even'>$admission_status</td>
</tr>":"")."
<tr>
<td class='head'>"._PICAL_TH_LASTMODIFIED."</td>
<td class='even'>$last_modified</td>
</tr>
<tr>
<td align='right'>
$edit_button
</td>
<td align='left'>
$ics_output_button
</td>
</tr>
As it is simple code, you can hack easily.