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

third_party/WebKit/WebCore/rendering/RenderThemeChromiumLinux.h

 /*
  * This file is part of the WebKit project.
  *
  * Copyright (C) 2006 Apple Computer, Inc.
  * Copyright (C) 2006 Michael Emmel mike.emmel@gmail.com
  * Copyright (C) 2007 Holger Hans Peter Freyther
  * Copyright (C) 2007 Alp Toker <alp@atoker.com>
  * Copyright (C) 2008, 2009 Google, Inc.
  * All rights reserved.
  * Copyright (C) 2009 Kenneth Rohde Christiansen
@@ skipping 36 matching lines @@
         virtual Color activeListBoxSelectionBackgroundColor() const;
         virtual Color activeListBoxSelectionForegroundColor() const;
         virtual Color inactiveListBoxSelectionBackgroundColor() const;
         virtual Color inactiveListBoxSelectionForegroundColor() const;
 
         virtual void adjustSliderThumbSize(RenderObject*) const;
 
         void setCaretBlinkInterval(double interval);
         virtual double caretBlinkIntervalInternal() const;
 
+        static void setScrollbarColors(unsigned inactive_color,
+                                       unsigned active_color,
+                                       unsigned track_color);
+        static unsigned thumbInactiveColor() { return m_thumbInactiveColor; }
+        static unsigned thumbActiveColor() { return m_thumbActiveColor; }
+        static unsigned trackColor() { return m_trackColor; }
+
     private:
         RenderThemeChromiumLinux();
         virtual ~RenderThemeChromiumLinux();
 
         // A general method asking if any control tinting is supported at all.
         virtual bool supportsControlTints() const;
 
         double m_caretBlinkInterval;
+
+        static unsigned m_thumbInactiveColor;
+        static unsigned m_thumbActiveColor;
+        static unsigned m_trackColor;
     };
 
 } // namespace WebCore
 
 #endif // RenderThemeChromiumLinux_h