.red::before {
content: 'red';
color: red;
}

.green::before {
content: 'green';
color: green;
}
$('p').removeClass('red').addClass('green');

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

// Create a new style tag
var style = document.createElement("style");

// Append the style tag to head
document.head.appendChild(style);

// Grab the stylesheet object
sheet = style.sheet

// Use addRule or insertRule to inject styles
sheet.addRule('.red::before','color: green');
sheet.insertRule('.red::before { color: green }', 0);

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

<p class="red" data-attr="red">Hi, this is plain-old, sad-looking paragraph tag.</p>
.red::before {
content: attr(data-attr);
color: red;
}
$('.red').attr('data-attr', 'green');