According to a comment in tikicon, that implementation can't work in Tikiwiki without breaking compatibility. Apparently ))TikiPro(( has incorporated it, however.
The basic idea of icon themes is to allow you, the admin, to change the icons that are used to navigate your site with out having to change all the templates or maintain a theme. You want heart themed icons, just create or download an icon directory (more on that to come), drop it is the img folder, and select it from the features admin panel. ))Ta-Da!(( - all your icons change.
Even better, you don't have to replace or duplicate every icon, just the ones you want! The rest will fall back to the defaults.
But wait! That's not all. For all you themers out there, you can change the icons for the whole site to better match your look with out changing every template! That's right - just drop an icon folder in your styles/<themename>/ directory and if the admin picks your theme, they get your icons as well. All without changing all the templates!
So how do I get all this wonderful functionality. It's all brought to you by the smarty function tikicon! Comming soon to a Tiki 1.9 near you!