On Linux, pick the color of the slider's thumbpart from the GTK theme....

third_party/WebKit/WebCore/rendering/RenderThemeChromiumLinux.cpp

 /*
  * Copyright (C) 2007 Apple Inc.
  * Copyright (C) 2007 Alp Toker <alp@atoker.com>
  * Copyright (C) 2008 Collabora Ltd.
  * Copyright (C) 2008, 2009 Google Inc.
  * Copyright (C) 2009 Kenneth Rohde Christiansen
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
@@ skipping 14 matching lines @@
 #include "config.h"
 #include "RenderThemeChromiumLinux.h"
 
 #include "Color.h"
 #include "CSSValueKeywords.h"
 #include "RenderObject.h"
 #include "UserAgentStyleSheets.h"
 
 namespace WebCore {
 
+unsigned RenderThemeChromiumLinux::m_thumbInactiveColor = 0xF0EBE5;
+unsigned RenderThemeChromiumLinux::m_thumbActiveColor = 0xFAF8F5;
+unsigned RenderThemeChromiumLinux::m_trackColor = 0xE3DDD8;
+
 PassRefPtr<RenderTheme> RenderThemeChromiumLinux::create()
 {
     return adoptRef(new RenderThemeChromiumLinux());
 }
 
 PassRefPtr<RenderTheme> RenderTheme::themeForPage(Page* page)
 {
     static RenderTheme* rt = RenderThemeChromiumLinux::create().releaseRef();
     return rt;
 }
@@ skipping 70 matching lines @@
 void RenderThemeChromiumLinux::setCaretBlinkInterval(double interval)
 {
     m_caretBlinkInterval = interval;
 }
 
 double RenderThemeChromiumLinux::caretBlinkIntervalInternal() const
 {
     return m_caretBlinkInterval;
 }
 
+void RenderThemeChromiumLinux::setScrollbarColors(
+    SkColor inactive_color, SkColor active_color, SkColor track_color)
+{
+    m_thumbInactiveColor = inactive_color;
+    m_thumbActiveColor = active_color;
+    m_trackColor = track_color;
+}
+
 } // namespace WebCore