you could use a fixed background , http://codepen.io/anon/pen/zGIrf
#chatroom {
position: relative;
height:50px;
overflow: scroll;
background:linear-gradient(to bottom,
blue 0 ,
pink 50px) no-repeat top;/*linear-gradient() can be replaced by an image url(myImage) to include older browser */
background-size:100% 50px;
background-attachment:fixed;
}
.chatmessage:nth-child(2n) {
background: #EEE;
}