How can I send a bootstrap table by email in php without losing format?

I am trying to generate a report, I recover my data from the database and I form a table with it, I want to output it by means of an email in php

mail($to,"Reportex","",$headers);

The problem is the bootstrap format is lost.

here the code:

$correos ="Bcc: [email protected]\r\n";
    $headers = "From: remitente>\r\n";
    $headers .= $correos;
    $headers .= "MIME-Version: 1.0\r\n";
    $boundary = uniqid("HTMLEMAIL");
    $headers .= "Content-Type: multipart/alternative;".
                "boundary = $boundary\r\n\r\n"; 
    $headers .= "This is a MIME encoded message.\r\n\r\n"; 
    $headers .= "--$boundary\r\n".
                "Content-Type: text/plain; charset=utf-8\r\n".
                "Content-Transfer-Encoding: base64\r\n\r\n"; 
    $headers .= chunk_split(base64_encode(strip_tags($content)));
    $headers .= "--$boundary\r\n".
                "Content-Type: text/html; charset=utf-8\r\n".
                "Content-Transfer-Encoding: base64\r\n\r\n"; 
    $headers .= chunk_split(base64_encode($content));
    mail($correos,"Reporte Seguimiento Productividad: ".$fecha1,"",$headers);