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

third_party/WebKit/WebCore/rendering/RenderThemeChromiumLinux.h

Index: third_party/WebKit/WebCore/rendering/RenderThemeChromiumLinux.h
===================================================================
--- third_party/WebKit/WebCore/rendering/RenderThemeChromiumLinux.h	(revision 51794)
+++ third_party/WebKit/WebCore/rendering/RenderThemeChromiumLinux.h	(working copy)
@@ -54,6 +54,13 @@
         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();
@@ -62,6 +69,10 @@
         virtual bool supportsControlTints() const;
 
         double m_caretBlinkInterval;
+
+        static unsigned m_thumbInactiveColor;
+        static unsigned m_thumbActiveColor;
+        static unsigned m_trackColor;
     };
 
 } // namespace WebCore