var link,
src = 'mystyle.css';
function addStyle() {
if(!link) {
link = document.createElement('link');
link.rel = 'stylesheet';
link.href = src;
link.type = 'text/css';
document.head.appendChild(link);
}
};
function removeStyle() {
if(link) {
document.head.removeChild(link);
link = null;
}
};
<button onclick="addStyle()">APPLY STYLE</button>
<button onclick="removeStyle()">CLEAR STYLE</button>