Changing the line spacing/line height in gtk+ apps

I am using Claws Mail (GTK+ based email application) and want to change the line spacing. Sadly there is no option for this in Claws itself.

There is a way to change the Font in the "~/.gtkrc"-file. Maybe somebody here know how to change the Line height too?
http://www.claws-mail.org/faq/index.php/Interface#How_to_change_the_font_size_in_the_Claws_Mail_interface.3F