border-bottom-width

значения свойства:

thin
medium
thick
value
inherit

css/style.css

* {
margin: 0;
padding: 0;
}
.main {
height: 600px;
width: 900px;
position: absolute;
top: 50%;
left: 50%;
margin-top: -300px;
margin-left: -450px;
}
.wrapper {
width: 33.3%;
height: 100%;
float: left;
}
#div1, #div2, #div3 {
height: 200px;
width: 200px;
float: left;
position: relative;
top: 50%;
left: 50%;
margin-top: -100px;
margin-left: -100px;
background-color: #C0C0C0;
border-bottom: solid #00F;
}
#div1 {
border-bottom-width: 1px;
}

* {
 margin: 0;
 padding: 0;
}

.main {
 height: 600px;
 width: 900px;
 position: absolute;
 top: 50%;
 left: 50%;
 margin-top: -300px;
 margin-left: -450px;
}

.wrapper {
 width: 33.3%;
 height: 100%;
 float: left;
}

#div1, #div2, #div3 {
 height: 200px;
 width: 200px;
 float: left;
 position: relative;
 top: 50%;
 left: 50%;
 margin-top: -100px;
 margin-left: -100px;
 background-color: #C0C0C0;
 border-bottom: solid #00F;
} 

#div1 {
 border-bottom-width: 1px;
}

index.html

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<link href="css/style.css" rel="stylesheet" />
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
</head>
<body>
<div class="main">
<div class="wrapper">
<div id="div1"></div>
</div>
<div class="wrapper">
<div id="div2"></div>
</div>
<div class="wrapper">
<div id="div3"></div>
</div>
</div>
<script>
/*JavaScript*/
document.getElementById("div2").style.borderBottomWidth = "medium";
/*jQuery*/
$(document).ready(function () {
$("#div3").css({ "border-bottom-width": "thick" });
});
</script>
</body>
</html>

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<link href="css/style.css" rel="stylesheet" />
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
</head>
<body>
<div class="main">
<div class="wrapper">
<div id="div1"></div>
</div>
<div class="wrapper">
<div id="div2"></div>
</div>
<div class="wrapper">
<div id="div3"></div>
</div>
</div>
<script>
/*JavaScript*/
document.getElementById("div2").style.borderBottomWidth = "medium";
/*jQuery*/
$(document).ready(function () {
$("#div3").css({ "border-bottom-width": "thick" });
});
</script>
</body>
</html>