how can add bg color to my header

var r1 = document.createElement("tr");
var h2 = document.createElement("th");
                        var a2 = document.createElement("a");
                       // a2.setAttribute("href","JavaScript:none{0};");
                       // a2.setAttribute("onclick","SortByName()");
                        a2.appendChild(document.createTextNode("DeptID"));
                        h2.appendChild(a2);
                        r1.appendChild(h2);
                         tb.appendChild(r1);