c# sending html email template include css not working

i trying to send html email template, it works perfect, but when i put css internal styles it gives error : input string was not in correct format,

my html template that works good:

<h3>data</h3>
<hr />
<table class="table table-striped table-bordered table-advance table-hover">
    <tr>
        <td>data </td>
        <td>{0}</td>
    </tr>
    <tr>
        <td>data </td>
        <td>{1}</td>
    </tr>
    <tr>
        <td>data</td>
        <td>{2}</td>
    </tr>
    <tr>
        <td>data</td>
        <td>{3}</td>
    </tr>
    <tr>
        <td>data</td>
        <td>{4}</td>
    </tr>
    <tr>
        <td>data</td>
        <td>{5}</td>
    </tr>
    <tr>
        <td>data</td>
        <td>{6}</td>
    </tr>
</table>

my html template that gives Input string was not in a correct format error :

<h3>data</h3>
<hr />
<table class="table table-striped table-bordered table-advance table-hover">
    <tr>
        <td>data </td>
        <td>{0}</td>
    </tr>
    <tr>
        <td>data </td>
        <td>{1}</td>
    </tr>
    <tr>
        <td>data</td>
        <td>{2}</td>
    </tr>
    <tr>
        <td>data</td>
        <td>{3}</td>
    </tr>
    <tr>
        <td>data</td>
        <td>{4}</td>
    </tr>
    <tr>
        <td>data</td>
        <td>{5}</td>
    </tr>
    <tr>
        <td>data</td>
        <td>{6}</td>
    </tr>
</table>
<style type="text/css">
    table td{border:solid 1px;}
</style>

my c# code that read and send:

C_GeneralSettings settings = db.C_GeneralSettings.SingleOrDefault();
S_ResidencesReservations item = db.S_ResidencesReservations.SingleOrDefault(x => x.ReserveId == entity.ReserveId);
string subject = "data";
string message = System.IO.File.ReadAllText(Server.MapPath("~/EmailTemplates/ReservationDetails.html"));
message = string.Format(message, entity.ReserveId, entity.IsApproved ? "yes" : "no", entity.PayementPeriodEndDate, entity.ReservationDate.ToString("MM/dd/yyyy"), entity.Name, entity.Email, entity.CellPhone);

HelperMethods.SendEmail(message, entity.Email, subject, settings);

and my send mail method:

public static void SendEmail(string messageContent, string toEmail, string subject, C_GeneralSettings emailEntity)
{
    //Check if email service is allowed
    bool allowEmailService = emailEntity.AllowEmailService;
    if (!allowEmailService)
        return;

    //Construct mail message
    string from = emailEntity.Email;
    string displayName = emailEntity.DisplayName;
    string to = toEmail;

    MailMessage message = new MailMessage();
    message.From = new MailAddress(from, displayName);
    message.To.Add(new MailAddress(to));
    message.Subject = subject;
    message.IsBodyHtml = true;
    message.Body = messageContent;

    //Get mail settings from config file
    string host = emailEntity.Host;
    string username = emailEntity.Email;
    string password = emailEntity.Password;
    int port = emailEntity.Port;
    bool enableSsl = emailEntity.EnableSSL;

    //Send the message
    SmtpClient smtp = new SmtpClient();
    smtp.Host = host;
    smtp.Port = port;
    smtp.Credentials = new System.Net.NetworkCredential(username, password);
    smtp.EnableSsl = enableSsl;

    smtp.Send(message);
}