Because you can't easily find a stylesheet in XUL by it's ID you have to look for the data attribute and compare that with what I created as the `css` variable.

test-stylesheet.js

function findStylesheet(document, name) {
  var uri = data.url(name + ".css"),
      css = "href=\"" + uri + "\" type=\"text/css\"",
      ORDERED_NODE_SNAPSHOT_TYPE = 7, // XPathResult doesn't exist in context
      xpath = null;

  xpath = document.evaluate("//processing-instruction(\"xml-stylesheet\")",
                            document,
                            document.createNSResolver(document),
                            ORDERED_NODE_SNAPSHOT_TYPE,
                            null);

  for (var i = 0; xpath && i < xpath.snapshotLength; i += 1) {
    if (xpath.snapshotItem(i).data === css) {
      return true;
    }
  }
  return false;
}