动态加css

document.getElementById('myElement').style.color = 'red';

var style = document.createElement('style');
style.type = 'text/css';
style.innerHTML = '#myElement { color: red; }';
document.getElementsByTagName('head')[0].appendChild(style);

var link = document.createElement('link');
link.rel = 'stylesheet';
link.href = 'path/to/your/stylesheet.css';
document.getElementsByTagName('head')[0].appendChild(link);

window.onload = function() {
    var link = document.createElement('link');
    link.rel = 'stylesheet';
    link.href = 'path/to/your/stylesheet.css';
    document.getElementsByTagName('head')[0].appendChild(link);
};