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

chrome/common/platform_util_linux.cc

Index: chrome/common/platform_util_linux.cc
===================================================================
--- chrome/common/platform_util_linux.cc	(revision 34028)
+++ chrome/common/platform_util_linux.cc	(working copy)
@@ -10,6 +10,7 @@
 #include "base/process_util.h"
 #include "base/singleton.h"
 #include "base/string_util.h"
+#include "chrome/browser/gtk/gtk_theme_provider.h"
 #include "chrome/common/gtk_util.h"
 #include "chrome/common/process_watcher.h"
 #include "googleurl/src/gurl.h"
@@ -88,11 +89,12 @@
   gtk_widget_show_all(dialog);
 }
 
-RendererPreferences GetInitedRendererPreferences() {
+RendererPreferences GetInitedRendererPreferences(Profile* profile) {
   RendererPreferences* prefs = Singleton<RendererPreferences>::get();
   static bool inited = false;
   if (!inited) {
-    gtk_util::InitRendererPrefsFromGtkSettings(prefs);
+    gtk_util::InitRendererPrefsFromGtkSettings(prefs,
+      GtkThemeProvider::GetFrom(profile)->UseGtkTheme());
     inited = true;
   }
   return *prefs;