My GTK 2.0 theme for Pidgin is still under review on freshmeat, so I have posted it here for downloading as well.
Windows zip: GTK2-TheMatrix.zip
gzip tarball: GTK2-TheMatrix.tar.gz
Unzip the contents into a new "TheMatrix" subfolder in your GTK themes path. On windows, this is C:\Program Files\Common Files\GTK\2.0\share\themes\
Next, if you are using the stock GTK+ that is included with the Pidgin installer, you will need to run the GTK themes selector. In the Windows Start menu: Start > All Programs > GTK+ > Theme Selector. Then select "TheMatrix" under either the current user or globally. (If you chose globally, make sure to also select it or '[always follow global settings]' under current user.)
If you are currently running Pidgin, close and reopen the application to apply the new theme.
In Pidgin go to the main menu Tools > Preferences, and the "Conversations" tab, make sure that under Font "Use font from theme" is checked.
Also, under Tools > Plugins you may want to at least initially disable the "Conversation Colors" and "Pidgin GTK+ Theme Control" plugins, as these can override the GTK settings for font, color and other settings for specific text elements within Pidgin.
If you are willing to play around with the plugins you can add some nice affects that go above and beyond the stock GTK skin, for example in the "Conversation Colors" plugin I set the "Sent Messages" text color to a slightly darker shade of green, such as #009600, so that in your convo log you can easily see a contrast between your sent and received messages.
I also highly recommend that in the "Conversation Colors" plugin you check the options "Ignore incoming format", "Apply in IMs", and "Apply in Chats" to taste, or you will be at the mercy of your friend's color settings in their IM client. (hot pink text or font size 20, anyone?) If you don't want to override any of the GTK colors you can leave the "customize" options unchecked.