string path_to_default_css = HttpContext.Current.Server.MapPath("~/assets/test.css");
string cssdefault = sr.ReadToEnd();

cssdefault.Replace("#252830","#000000);

.slider-handle {
position: absolute;
width: 20px;
height: 20px;
background-color: #252830;
background-image: -webkit-linear-gradient(top, #252830 0%, #252830 100%);
background-image: -o-linear-gradient(top, #252830 0%, #252830 100%);
background-image: linear-gradient(to bottom, #252830 0%, #252830 100%);
}