function changeCSS(cssFile, cssLinkIndex) {
var oldlink = document.getElementsByTagName("link").item(cssLinkIndex);
var newlink = document.createElement("link");
newlink.setAttribute("rel", "stylesheet");
newlink.setAttribute("type", "text/css");
newlink.setAttribute("href", cssFile);
document.getElementsByTagName("head").item(0).replaceChild(newlink, oldlink);
}
<html>
<head>
<title>Changing CSS</title>
<link rel="stylesheet" type="text/css" href="positive.css"/>
</head>
<body>
<a href="#" onclick="changeCSS('positive.css', 0);">STYLE 1</a>
<a href="#" onclick="changeCSS('negative.css', 0);">STYLE 2</a>
</body>
</html>
<div class="main1">
<div class="sub"></div>
<div class="sub"></div>
<div class="sub"></div>
</div>
<div class="main2">
<div class="sub"> </div>
<div class="sub"> </div>
<div class="sub"> </div>
</div>
.main1 {
.sub { color: red }
}
.main2 {
.sub { color: blue }
}
.lineAboutUs:before{
content: "";
display: block;
height: 4px;
position: relative;
top: -20px;
background: -moz-linear-gradient( 0deg, rgb(112,151,209) 0%, rgb(38,62,102) 51%, rgb(255,255,255) 100%);
background-image: -webkit-linear-gradient( 0deg, rgb(112,151,209) 0%, rgb(38,62,102) 51%, rgb(255,255,255) 100%);
background-image: -ms-linear-gradient( 0deg, rgb(112,151,209) 0%, rgb(38,62,102) 51%, rgb(255,255,255) 100%);
}