Add CSS to MPDF

I am trying to add CSS to my MPDF , when I remove the lines from CSS works normally, I have my file CSS en in the same path of my PHP , it gives me errors with CSS this is my code:

$mpdf=new mPDF('c','A4');
$stylesheet = file_get_contents('pdf.css'); // la ruta a tu css

$mpdf->WriteHTML($stylesheet,1);

$mpdf->WriteHTML($html,2);
$mpdf->Output('reporte.pdf','I');