How to change an embedded CSS file before registering it?

Question

I'm writing a custom control library with some embedded CSS files. Some of the CSS classes holds references to images (like the .AlternateRow listed bellow). I want to deploy the images as embedded resources on my assembly but there is a problem: I only know the path to the embedded images on runtime. So my question is:

How could I change the class .AlternateRow in runtime, rigth before register it's css file?

The original CSS class...

.AlternateRow
{
    background-image: url('/images/control-bg.png');
    background-repeat: repeat-x;    
}

Must become something like

.AlternateRow
{
    background-image: url('/WebResource.axd?d=some-key;t=other-key');
    background-repeat: repeat-x;    
}