Директории:
nakedxulapp\chrome\skin\classic\nakedxulapp

chrome.manifest:
...
skin	nakedxulapp classic/1.0	jar:chrome/nakedxulapp.jar!/skin/classic/nakedxulapp/
...

mainWindow.xul:
...
<?xml-stylesheet href="chrome://nakedxulapp/skin/mainWindow.css" type="text/css"?>
...

nakedxulapp\chrome\skin\classic\nakedxulapp\mainWindow.css:
...
#itr-tbbCreate, #itr-cmCreate {
    list-style-image: url("chrome://nakedxulapp/skin/itr-create.gif");
}
...

<?xml-stylesheet href="chrome://nakedxulapp/skin/mainwindow/mainWindow.css" type="text/css"?>

#itr-tbbCreate, #itr-cmCreate {
    list-style-image: url("chrome://nakedxulapp/skin/mainwindow/itr-create.gif");
}