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

chrome/browser/tab_contents/tab_contents.cc

Index: chrome/browser/tab_contents/tab_contents.cc
===================================================================
--- chrome/browser/tab_contents/tab_contents.cc	(revision 34028)
+++ chrome/browser/tab_contents/tab_contents.cc	(working copy)
@@ -259,7 +259,8 @@
       last_javascript_message_dismissal_(),
       suppress_javascript_messages_(false),
       is_showing_before_unload_dialog_(false),
-      renderer_preferences_(platform_util::GetInitedRendererPreferences()),
+      renderer_preferences_(
+        platform_util::GetInitedRendererPreferences(profile)),
       opener_dom_ui_type_(DOMUIFactory::kNoDOMUI) {
 #if defined(OS_CHROMEOS)
   // Make sure the thumbnailer is started before starting the render manager.
@@ -1882,7 +1883,7 @@
   return autofill_manager_.get();
 }
 
-RendererPreferences TabContents::GetRendererPrefs() const {
+RendererPreferences TabContents::GetRendererPrefs(Profile* profile) const {
   return renderer_preferences_;
 }