You are right skolnick , and this is the feature I wish Developers put in the gui but I'm not a programmer and don't know if it's possible to change dinamically via aMule GUI the window GTK2 properties.
But I've Googled a little and I've found a solution althogh a little hacky

The GTK theme can be passed to a single application via environment variable, first you have to make a file like 'mytheme.gtkrc' with this:
#To set the Gtk theme
# To set the theme
#include "/home/yourname/.themes/exampletheme/theme1"
#To set the icon theme
#gtk-icon-theme-name = "Dropline Neu!"
#To set the font
style "Sans"
{
font_name = "Liberation Sans"
}
widget_class "*" style "Sans"
gtk-font-name = "Liberation Sans B"
In this example I have only changed the font to a narrower and bold one all the other options are commented.
Then you save this file in your/home/yourname/.aMule directory for instance, make it executable, and if you want to make a launcher you should put this in the command line:
sh -c 'GTK2_RC_FILES=/home/yourname/.aMule/mytheme.gtkrc amule'
and your aMule should start with narrower fonts.
Saludos/Greetz