I am not sure how to add those classes by anchoring an event to that selector but will look into that.
Using this pattern:
MktoForms2.whenRendered(function(form){
var formEl = form.getFormElem()[0],
arrayFrom = getSelection.call.bind([].slice);
arrayFrom(formEl.querySelectorAll(".mktoFormRow"))
.forEach(function(rowEl){
rowEl.className += " some-class-name";
});
});