Apply css to a popup

Hi
I'm building a selectbox insead of using the common select tag.
I use the windows.createpopup() method to display the list of choise.
Differently to the div object the popup object is like a new window so it doesn't inherit the css style sheet I've attached to the main window.
I tought to attach it by using the addImport(url) method of the styleSheet object but it doesn't seem to work. Here it's my code, the show function it's called when the selectbox expands:

var Select_box = window.createPopup();
Select_box.document.styleSheets[0].addImport("URL('common/css/default_style.css');")

function show(element) {
if (Select_box.isOpen==false) {
Select_box.document.body.innerHTML = Select_box_content.innerHTML;
Select_box.document.body.className='selectbox';
Select_box.show(0, 18, 150, 50, element);
}
else {
Select_box.hide();
}
}

I've noticed that using the addImport method on the main window, and not on the instance of the select_box, with 0 index of the styleSheet the error doesn't occur but it doesn't also change the proprety. Tring using any other index value it gives me the usual error.
So how can I attach a style sheet to a popup created with the window.createpopup()?

Thank you