I already succeed to added the transition effect when hover the cursor to drop-down menu (click the "M" logo) in my own wordpress theme but when the cursor being away from drop-down menu don't show the transition effect when closing. You can check out my own theme on this link for further info.
Here drop-down css style:
.dropdown-menu {
border-bottom-right-radius: 77px;
border-bottom-left-radius: 77px;
/* background: url(img/submenu.png) no-repeat scroll right/ 91% 100%; */
position: absolute;
height:0;
top: 100%;
left: 0;
z-index: 1000;
display: none;
float: left;
min-width: 153px;
padding: 1px 0;
margin: 2px 0 0 0;
list-style: none;
font-size: 14px;
text-align: center;
background-color: transparent!important;
border: 1px solid #ccc;
border: 1px solid rgba(0,0,0,0.15);
background-clip: padding-box;
opacity: 0;
top: 35px;
visibility: hidden;
-webkit-transition:height 300ms ease-in;
-moz-transition:height 300ms ease-in;
-ms-transition: height 300ms ease-in;
-o-transition:height 300ms ease-in;
transition:height 300ms ease-in;
overflow: hidden;
}
.navbar-nav li:hover .dropdown-menu {
opacity: 1;
top: 105px;
visibility: visible;
height:300px;
}
So How I can add the transition effect when drop-down menu closing?