diff -r 000000000000 -r 6474c204b198 browser/devtools/shared/theme-switching.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/devtools/shared/theme-switching.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,88 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+(function() {
+  const DEVTOOLS_SKIN_URL = "chrome://browser/skin/devtools/";
+
+  function forceStyle() {
+    let computedStyle = window.getComputedStyle(document.documentElement);
+    if (!computedStyle) {
+      // Null when documentElement is not ready. This method is anyways not
+      // required then as scrollbars would be in their state without flushing.
+      return;
+    }
+    let display = computedStyle.display; // Save display value
+    document.documentElement.style.display = "none";
+    window.getComputedStyle(document.documentElement).display; // Flush
+    document.documentElement.style.display = display; // Restore
+  }
+
+  function switchTheme(newTheme, oldTheme) {
+    if (newTheme === oldTheme) {
+      return;
+    }
+
+    if (oldTheme && newTheme != oldTheme) {
+      StylesheetUtils.removeSheet(
+        window,
+        DEVTOOLS_SKIN_URL + oldTheme + "-theme.css",
+        "author"
+      );
+    }
+
+    StylesheetUtils.loadSheet(
+      window,
+      DEVTOOLS_SKIN_URL + newTheme + "-theme.css",
+      "author"
+    );
+
+    // Floating scrollbars à la osx
+    let hiddenDOMWindow = Cc["@mozilla.org/appshell/appShellService;1"]
+                 .getService(Ci.nsIAppShellService)
+                 .hiddenDOMWindow;
+    if (!hiddenDOMWindow.matchMedia("(-moz-overlay-scrollbars)").matches) {
+      let scrollbarsUrl = Services.io.newURI(
+        DEVTOOLS_SKIN_URL + "floating-scrollbars-light.css", null, null);
+
+      if (newTheme == "dark") {
+        StylesheetUtils.loadSheet(
+          window,
+          scrollbarsUrl,
+          "agent"
+        );
+      } else if (oldTheme == "dark") {
+        StylesheetUtils.removeSheet(
+          window,
+          scrollbarsUrl,
+          "agent"
+        );
+      }
+      forceStyle();
+    }
+
+    document.documentElement.classList.remove("theme-" + oldTheme);
+    document.documentElement.classList.add("theme-" + newTheme);
+  }
+
+  function handlePrefChange(event, data) {
+    if (data.pref == "devtools.theme") {
+      switchTheme(data.newValue, data.oldValue);
+    }
+  }
+
+  const { classes: Cc, interfaces: Ci, utils: Cu } = Components;
+
+  Cu.import("resource://gre/modules/Services.jsm");
+  Cu.import("resource:///modules/devtools/gDevTools.jsm");
+  const {devtools} = Components.utils.import("resource://gre/modules/devtools/Loader.jsm", {});
+  const StylesheetUtils = devtools.require("sdk/stylesheet/utils");
+
+  let theme = Services.prefs.getCharPref("devtools.theme");
+  switchTheme(theme);
+
+  gDevTools.on("pref-changed", handlePrefChange);
+  window.addEventListener("unload", function() {
+    gDevTools.off("pref-changed", handlePrefChange);
+  });
+})();