[ruslan_mart]

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>