slide-button-background.css

.btn{
    box-shadow: inset 0 0 0 0 #31302B;
	-webkit-transition: all ease 0.8s;
	-moz-transition: all ease 0.8s;
	transition: all ease 0.8s;
}
.btn{
    box-shadow: inset 100px 0 0 0 #fff;
}