Sorry Mary,
I managed to accomplish what I wanted. I create a toolbar button accessibility with high contrast. it set the click event call one jquery function that makes a call to a php file which sets a cookie. In renderer.php put a test file with a conditional structure 'if' in order to check the existence of a cookie set when calling the file that call in jquery with $. Get. If by chance the cookie exists it includes the css file that I want. So I load the CSS dynamically.
into renderer:
if (isset($cookie['contrast'])){
$cssurl = '/theme/themename/style/contrast.php';
$PAGE->requires->css($cssurl);
}
into contrast.php:
header('Content-Type: text/css');
echo '* {color: #ffff00 !important;}
* {background-color: #000 !important;}
* {background-image: none !important;}
#content a, .tabrow0 span {color: #ff0 !important;}
.tabrow0 span:hover {text-decoration: underline;}
.block_accessibility .outer {border-color:#fff !important;}';
note: To use a cookie on moodle have to do the following:
require('/../../../../config.php');
global $CFG;
setcookie('somevalue', time()+(DAYSECS*60), $CFG->sessioncookiepath, $CFG->sessioncookiedomain, $CFG->cookiesecure, $CFG->cookiehttponly);//set cookie
setcookie('somevalue', time()-(DAYSECS*60), $CFG->sessioncookiepath, $CFG->sessioncookiedomain, $CFG->cookiesecure, $CFG->cookiehttponly);//unset cookie
thank for your help