Maybe because it's not function but method in class (something with OOP). And it uses new way of creating forms (admin_ui functionality). You can try to use type method for some field and write method for it, but I am really not sure if this can be used in theme preferencies. They are just fields, text, bbarea, anything but you need to set them correct way. Each field can help help value when you can use html code.
I still don't know what code you want to use there, but theme preferencies are for setting values, so they are just only form elements. It's a type of form, nothing more.