Why my CSS is not working with the XUL app?

My xul app:

<?xml version="1.0"?>
<?xml-stylesheet href="main.css" type="text/css"?>

<window id="main" title="MY TEST" width="640" height="480" xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
    <listbox id="mainList">
      <listitem label="Butter Pecan"/>
      <listitem label="Chocolate Chip"/>
      <listitem label="Raspberry Ripple"/>
      <listitem label="Squash Swirl"/>
    </listbox>
</window>

The main.css:

body {background-color:#b0c4de;}

and the result is a transparent window, why the color is not filling?

I know nothing about XUL, but I would assume that the style isn't working because there isn't a body tag in your XUL code.

Try this:

#main {background-color:#b0c4de;}