What needs to be done is to copy the humanity/categories/XX/application-fonts icon to the hicolor (or gnome) icon set.
Because hicolor is the default fallback theme for all iconsets. So, older themes (any theme on gnome-look, really) that do not have an application-fonts icon, should be able to find it in hicolor.
Actually, this must be done with all new icons added to the humanity/categories folder.
What needs to be done is to copy the humanity/ categories/ XX/application- fonts icon to the hicolor (or gnome) icon set.
Because hicolor is the default fallback theme for all iconsets. So, older themes (any theme on gnome-look, really) that do not have an application-fonts icon, should be able to find it in hicolor.
Actually, this must be done with all new icons added to the humanity/categories folder.