You just bind them:
$('#<%=lblMessage.ClientID%>')
.css("color", "#16428b")
.css("font-family", "Helvetica, Arial, sans-serif")
.css("background", "#ccc");
jQuery jQuery, .
Edit:
, , , :
var e = document.getElementById('<%=lblMessage.ClientID%>');
e.style.color = '#16428b';
e.style.fontFamily = 'Helvetica, Arial, sans-serif';
e.style.backgroundColor = '#ccc';