I'm not quite sure what you are after, ie I don't know where you want the icons, I've done my best to tell how to use them in 2.3.
In prefixes you can use this in extra.less for before and/ or after, e.g. with the lightGreen prefix:
Code:
.label.label--lightGreen {
&:before {
.m-faContent(@fa-var-flower-tulip);
}
&:after {
.m-faContent(@fa-var-flower-daffodil);
}
}