Pseudo-element CSS dynamically using JavaScript

Is it possible to set the CSS of a pseudo-element dynamically? For example:

jQuery dynamically styling help container

$('#help').css({
    "width" : windowWidth - xOffset,
    "height" : windowHeight - yOffset,
    "bottom" : -windowHeight,
    "left" : 200
});

jQuery Attempt at setting the inner border of the help container:

$('#help:before').css({
    "width" : windowWidth - xOffset,
    "height" : windowHeight - yOffset
});

CSS file for the above

#help
{
    opacity: 0.9;
    filter:alpha(opacity=90);   
    -moz-opacity: 0.9;          
    z-index: 1000000;
    bottom: -550px;
    left: 400px;
    background-color: #808080;
    border: 5px dashed #494949;
    -webkit-border-radius: 20px 20px 20px 20px;
    -moz-border-radius: 20px 20px 20px 20px;
    开发者_StackOverflowborder-radius: 20px 20px 20px 20px;        
}
#help:before 
{
    border: 5px solid white;
    content: '';
    position: absolute;
    -webkit-border-radius: 20px 20px 20px 20px;
    -moz-border-radius: 20px 20px 20px 20px;
    border-radius: 20px 20px 20px 20px;          
}

You can't do it directly through jQuery.

Look at this question: Setting CSS pseudo-class rules from JavaScript

@Box9's answer is probably the one you should actually use:

I threw together a small library for this since I do think there are valid use cases for manipulating stylesheets in JS.

Rather than setting the width and height using .css(), you should just use .width() and height() directly.