On a fresh install of Debian Wheezy, I have Leafpad 0.8.18.1-3. Following instructions I created a .gtkrc-2.0 in my home folder and put the commands to change the text area colors. This had no effect.
The problem has been noted before, but didn't get an answer. I did find mention of someone having it work several years ago.
If you use a Desktop Environment, your .gtkrc-2.0 may be totally ignored. In Xfce4 for example, you can add rules manually through xfce4-settings-editor, under xsettings -> Gtk.
You can also try editing the gtkrc file for the Gtk+ theme you are currently using. It is under /usr/share/themes/THEMENAME/gtk-2.0/gtkrc or ~/.themes/THEMENAME/gtk-2.0/gtkrc
