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

third_party/WebKit/WebKit/chromium/src/WebViewImpl.cpp

Index: third_party/WebKit/WebKit/chromium/src/WebViewImpl.cpp
===================================================================
--- third_party/WebKit/WebKit/chromium/src/WebViewImpl.cpp	(revision 51794)
+++ third_party/WebKit/WebKit/chromium/src/WebViewImpl.cpp	(working copy)
@@ -97,6 +97,9 @@
 #include "KeyboardCodesWin.h"
 #include "RenderThemeChromiumWin.h"
 #else
+#if PLATFORM(LINUX)
+#include "RenderThemeChromiumLinux.h"
+#endif
 #include "KeyboardCodesPosix.h"
 #include "RenderTheme.h"
 #endif
@@ -1621,6 +1624,16 @@
     return (page() && page()->focusController()) ? page()->focusController()->isActive() : false;
 }
 
+void WebViewImpl::setScrollbarColors(unsigned inactive_color,
+                                     unsigned active_color,
+                                     unsigned track_color) {
+#if PLATFORM(LINUX)
+    RenderThemeChromiumLinux::setScrollbarColors(inactive_color,
+                                                 active_color,
+                                                 track_color);
+#endif
+}
+
 void WebViewImpl::didCommitLoad(bool* isNewNavigation)
 {
     if (isNewNavigation)