how do I include my css file in a ascx file.

I got a few lines of css
#UserAvailability {
  padding-left: 22px;
  background-position: left;
  background-repeat: no-repeat;
}
 
.taken {
  background-image: url(taken.gif);
}
 
.available {
  background-image: url(available.gif);
}
how should I include this file.

Since I have a master page that includes defaults.css

should i put it in default.css and reference, if so how can I do this.
if not what should i do?

my file is:
using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using OmegaLove;
using OmegaLove.UI;
using App_Code.DAL;
using App_Code.BLL;


namespace Controls
{
    public partial class Register : System.Web.UI.UserControl
    {

     

        protected void Page_Load(object sender, EventArgs e)
        {


            if (!Page.IsPostBack)
            {

                cddCountryRegister.SelectedValue = "CA";
                imgFlagRegister.ImageUrl = "~/images/flags/" + cddCountryRegister.SelectedValue + ".gif";

            }
        }

        protected void imgRegister_Click(object sender, ImageClickEventArgs e)
        {
            Response.Write("HELLO");
        }
       
        protected void Username_Changed(object sender, EventArgs e)
        {
            if (Membership.GetUser(txtUserName.Text) != null)
            {
                UserAvailability.InnerText = "Username taken, sorry.";
                UserAvailability.Attributes.Add("class", "taken");
            }
            else
            {
                UserAvailability.InnerText = "Username available!";
                UserAvailability.Attributes.Add("class", "available");
            }
        }
}

}
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