@Milan-Labudovic, Joseph may or may not have answered your question.
If you're using a Cloud Backend function for form submission, you can add custom javascript for each value, or execute custom javascript script before or after submitting to CloudBackend.
I do this myself 🙂 See below.
Click on your submit button -> Action -> Cloud Backend ->
Screenshot 2020-08-04 22.03.05(2).png
If you are talking about custom CSS, maybe you can add a custom class you define elsewhere, or some custom styles here:
Brush icon -> Identification (Advanced) ->
Screenshot 2020-08-04 22.08.14.png