Underline active menu button

Hi there!
Can anybody tell me which css code to use in order to have the active menu button underlinded?
Thanks for your help!

I hope this code will help you, try this.

a {
  text-decoration: none;

	text-align: center;

div#top_nav li{
	display: inline;
	padding: 15px;

div#top_nav a {

display: inline-block;
position: relative;


div#top_nav a:hover{
	color: orange;

div#top_nav a:before{

  content: "";
  position: absolute;
  width: 100%;
  height: 2px;
  bottom: -3px;
  left: 0;
  background-color: orange;
  visibility: hidden;
  -webkit-transform: scaleX(0);
  transform: scaleX(0);
  -webkit-transition: all 0.15s ease-in-out 0s;
  transition: all 0.15s ease-in-out 0s;


div#top_nav a:hover:before , div#top_nav a.active:before {
  visibility: visible;
  -webkit-transform: scaleX(1);
  transform: scaleX(1);

.active {
	color: orange;