tansparent Panorama background for https://bugzilla.mozilla.org/show_bug.cgi?id=593388

panorama-lwtheme.uc.js

// userChorme.js
// @see https://bugzilla.mozilla.org/show_bug.cgi?id=593388

window.addEventListener("tabviewframeinitialized", function (event) {
  this.removeEventListener(event.type, arguments.callee, false);
  var win = TabView._window;
  var tabViewRootElement = win.document.documentElement;

  function updateBackground (isLwthemeActive) {
    var imageStyle = "";
    if (isLwthemeActive) {
      let bgColor = window.getComputedStyle(document.documentElement, "").backgroundColor;
      imageStyle = "-moz-linear-gradient(center top, transparent, transparent 150px," + bgColor + " 190px, " + bgColor + ")";
    }
    win.document.getElementById("bg").style.backgroundImage = imageStyle;
  }

  if (window.document.documentElement.hasAttribute("lwtheme")) {
    tabViewRootElement.setAttribute("lwtheme", "true");
    updateBackground(true);
  }

  var lwthemeObserver = {
    rootElement: tabViewRootElement,
    observe: function (aSubject, aTopic, aData) {
      if (aTopic != "lightweight-theme-styling-update")
        return;

      if (aData === "null") {
        this.rootElement.removeAttribute("lwtheme");
        updateBackground(false);
      } else {
        this.rootElement.setAttribute("lwtheme", "true");
        updateBackground(true);
      }
    },
  };
  win.Services.obs.addObserver(lwthemeObserver, "lightweight-theme-styling-update", false);
}, false);