One of the newer updates to Pidgin made it a little harder to use GTK+ themes in the application (I like mine to look like the window below). You can follow the instructions from this link to get Pidgin looking decent:
Once you’ve modified your Pidgin install, you will want to place your themes into
<Pidgin Installation Directory>\Gtk\share\themes. My favorite Pidgin/GTK theme is ClearLooks DarkLime. It’s awesome for both Pidgin and Ubuntu.