Blob to PDF not retaining HTML CSS

Question

I have the following HTML -

            htmlBody +=                     '<table width="100%" style="background:#16284c;max-width: 900px;margin: auto;">';
        htmlBody +=                         '<tr>';
        htmlBody +=                             '<td style="text-align: center; vertical-align: middle;">';
        htmlBody +=                                 '<H2 style="color:#ffffff;">PBM Partner Information</H2>';
        htmlBody +=                             '</td>';
        htmlBody +=                         '</tr>';
        htmlBody +=                     '</table>';

I convert the HMTL to PDF using the following:

Blob.toPDF(htmlBody);

Then I add the PDF to an email as an attachment.

   List<Messaging.EmailFileAttachment> attachments = new List<Messaging.EmailFileAttachment>();


    Messaging.EmailFileAttachment efa = new Messaging.EmailFileAttachment();

    efa.setBody(Blob.toPDF(htmlBody));
    efa.setContentType('application/pdf');
    efa.setFileName('test.pdf');
    attachments.add(efa);

    email.setFileAttachments(attachments);

The PDF loads, but the CSS is gone. Why?