From official Pidgin website: http://developer.pidgin.im/wiki/Using%20Pidgin#win32_gtk_theme
As of version 2.7.0, Pidgin ships with a private GTK+ runtime and doesn't include extra themes or a GUI theme switching application.
There is an easy way to use the GTK+ theme binaries and theme switcher that Alexander Shaduri maintains. Note that we don't recommend you use the GTK+ installer he distributes unless you know what you're doing and have a good reason for doing so.
Download the GTK+ Themes and GTK+ Theme Switcher zip files and extract them to a temporary directory.
Copy the lib and share directories from the GTK+ Themes zip file into your \Gtk directory (you want to merge those with the existing lib and share directories). Copy the gtk2_prefs.exe from the GTK Theme Switcher zip file into your \Gtk\bin directory.
You can now run the gtk2_prefs.exe applications to choose a theme. You probably will want to use the "Apply for all users" checkbox so that the changes are made to Pidgin's private GTK+ runtime.
If you follow their recommendation you will notice when you try and run the theme selector exe you will get tons of missing dll errors. I am looking around for solutions now and will post as soon as I find one that is simple for most users. What I had to go through to get this working correctly was not feasible by any standards.
*** If anyone has a fix I would love to see it, I can't get it working using any of Pidgins suggestions or any of my own tinkering.
Ticket posted at Pidgin: http://developer.pidgin.im/ticket/12091