set up a custom css one time and have it apply for all of my selected forms

It Is also possible to add a field in already existing multiple forms? Not 1 form at the time.

It would save a lot off time :-)

Unfortunately no, it is currently not possible to apply a custom css to many forms. Same for adding the same field to all forms. 

We are actually working on a new tool that will make it possible to create your own custom themes for jotform. Just like custom CSS. So, once we have that you will be able to use the same theme on many forms and that way apply the same custom CSSes to all of the forms.