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

third_party/WebKit/WebCore/rendering/RenderThemeChromiumLinux.cpp

Index: third_party/WebKit/WebCore/rendering/RenderThemeChromiumLinux.cpp
===================================================================
--- third_party/WebKit/WebCore/rendering/RenderThemeChromiumLinux.cpp	(revision 51794)
+++ third_party/WebKit/WebCore/rendering/RenderThemeChromiumLinux.cpp	(working copy)
@@ -32,6 +32,10 @@
 
 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());
@@ -122,4 +126,12 @@
     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