You talk about these, right?
Sorry, I still do not understand what you want to "override"? The look/texture of the target markers?
For example: Change the "skull target marker"
to something differently?
That's not possible as far as I know.
I had tried that, and it never worked in the past.
Neither changing the target marker's size (there maybe a setting by the vanilla game meanwhile to change the size? At least ZOs said once they might add on)
or color worked.
btw:
FCO TargetMakers is using the target markers of ZOs! There was and is no other way to apply markers above the enemies -> Only ZOs target markers.