How to change Font Style of JavaScript Alert Box using CSS

hi

I used below code for showing message in my page

    if (Session["Message"] != null)
        {

            //Write message code here
            this.ClientScript.RegisterStartupScript(GetType(), "Javascript", "<script>alert('insert succesfullyا')</script>");

            Session["Message"] = null;
        }

I want this message show with (font=Tahoma  font-size=12px and  font-weight:bold )

How I can do it?

Best Regards

Neda