cache.js

global.getStylesheets = () => {
  let styleSheetService = Cc["@mozilla.org/content/style-sheet-service;1"]
      .getService(Components.interfaces.nsIStyleSheetService);
  let styleSheetURI = Services.io.newURI("chrome://browser/content/extension.css",
                                         null, null);
  let styleSheet = styleSheetService.preloadSheet(styleSheetURI,
      styleSheetService.AGENT_SHEET);
  styleSheetURI = Services.io.newURI("chrome://browser/content/extension-mac.css",
      null, null);
  let macStyleSheet = styleSheetService.preloadSheet(styleSheetURI,
      styleSheetService.AGENT_SHEET);
  global.getStylesheets = () => {
    return {"base": styleSheet, "mac": macStyleSheet};
  };
  return global.getStylesheets();
};