Part 1:
[code]<?php
$acl =& JFactory::getACL();
jimport('joomla.application.component.helper'); // include libraries/application/component/helper.php
$usersParams = & JComponentHelper::getParams('com_users'); // load the Params
// "generate" a new JUser Object
$user = & JFactory::getUser();

$data = array($user); // array for all user settings
$usertype = $usersParams->get($user->id);
$user =& JFactory::getUser(); $usergid = $user->get('gid'); 
//echo "Group ID = " . $usergid . ' ' ; //I know the groupID now[/code]

Next step is to evaluate and write the var for use in the stylesheetname:
[code]
switch ($usergid) {
    case 31:
        //echo 'Groep = ' .$usergid . ', Herlecollege'; 
		$template ='hc_bedlam';
		$css= 'hc';
        break;
    case 32:
        //echo 'Groep = '. $usergid . ', Vrienden';
		$template= 'web_bedlam';
		$css= 'web';
    default: $usergid = 0;
		$template= 'base';
		$css = 'base';
		break;
}[/code]

The 'vars'  $template and $css are used in the following 


[code]<link rel="stylesheet" href="<?php echo $this->baseurl ?>/templates/<?php echo $template?>/css/template.css" type="text/css" /> 
<link rel="stylesheet" href="<?php echo $this->baseurl ?>/templates/<?php echo $template?>/css/<?php echo $css ?>.css" type="text/css" />[/code]