Disable form progression based radio selection

I am trying to disable the next button on a form based on the selection of a radio field. How can this be achieved? It would also be ideal if a warning message was displayed (similar to this field is required) as well.

You can execute this with display rules and CSS. Take a peek at the video that was completed on CSS and Display Rules.

I followed both this video and the updated video covering the same topic. One thing that I am noticing is when the default navigation buttons are disabled the form navigation breaks. Specifically, the number bubbles no longer highlight which page the user is currently on.

Also, is there a way to fade the buttons in and out when they appear/ disappear instead of the hard pop in? I am not great at CSS but know that transition: background-color 0.3s ease, color 0.3s ease can switch between colors. Does something similar exist for transitioning the elements in and out?