How much backend control can we implement in the upfront builder

In a normal Wordpress theme we can create a child theme and make code changes to the way pages and features work. Can this be done in upfront?

There are several things we would like to add to our upfront themes.

1) Add fastclick.js for better mobile responsiveness

2) triggered menu uses a display of "none" to handle the appearance and disappearance of the menu. This is not the most ideal use for showing and hiding the menu as the display none attribute can not be animated. Can we change this. We would like to change it to a visibility attribute and then set a position that changes via a css animation.

3) we would also like to add something like the ionic framework css to have better styling of inputs like radio boxes.