diff -r 000000000000 -r 6474c204b198 browser/themes/windows/devtools/floating-scrollbars-light.css
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/themes/windows/devtools/floating-scrollbars-light.css	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,10 @@
+/* vim:set ts=2 sw=2 sts=2 et: */
+/* 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/. */
+
+@import url("floating-scrollbars.css");
+
+scrollbar thumb {
+  background-color: rgba(170,170,170,0.2) !important;
+}