Styling Application button after activated

Hi there, I understand this may be out of the scope of your support, but hopefully you can point me in the right direction.

I have created a custom layout for the single posts and then have the application button set as sticky. However once clicked on the application fills the screen which is fine, but is there a way to style the application button, so after activated it can have a “X” or some kind of indication by clicking on it, it will close the application?

I have been trying, but can’t seem to get it working.

The page I need help with: [log in to see the link]