用转义序列连接两个字符串(Concatenating two strings with escape sequences)

 你可以在Selector之后使用css ::来创建这种类型的东西。 
 
.col-sm-2::after {
    content: "";
    position: absolute;
    width: 90%;
    height: 3px;
    left: 0;
    right: 0;
    margin: auto;
    background: #aaa;
    transition: all .35s;
}
.col-sm-2:hover::after {
    background: #68c122;
}
 
 这是一个小小的开始。 但我强烈建议使用<nav class="navbar" role="navigation"></nav>创建导航。 

You can use css ::after Selector for creating this type of thing. 
 
.col-sm-2::after {
    content: "";
    position: absolute;
    width: 90%;
    height: 3px;
    left: 0;
    right: 0;
    margin: auto;
    background: #aaa;
    transition: all .35s;
}
.col-sm-2:hover::after {
    background: #68c122;
}
 
Here is a small fiddle to get started. But I strongly suggest Using <nav class="navbar" role="navigation"></nav> for a creating navigation.