<p class="red">Hello,are you kain?</p>

.red:before {
  content: 'red',
  background-color : red
}

.green:before {
  content: 'green';
  background-color: green;
}

$('p').addClass('green');

document.styleSheet[0].addRule('.red:before','background-color:green');
document.styleSheet[0].insertRule('.red:before{background-color:green}',0);

var style = document.creatElement('style');
document.head.appendChild(style);
sheet = style.sheet;
sheet.addRule('.red:before','background-color:green');
sheet.insertRule('.red:before{background-color:green}',0);

// jQuery
$('<style>.red:before{background-color:green}</style>').appendTo('head');

$('red').attr('data-attr','green');