If you want to change the 'look' of the output screens your first choice should really be use a different CSS file, as documented in http://www.tonymarston.net/php-mysql/cascading-style-sheets. html
If you really must switch to a different XSL file at runtme then this is possible. The contents of the screen structure file is read into a global variable called $structure at the start of each script, so its contents can be amended before being used to generate the HTML output. You will need to put code into each script similar to the following:
if ($_SESSION['role_id'] == 'whatever') }
$GLOBALS['structure']['xsl_file'] = 'whatever';
}