Option 1/ As you suggested yourself.
Yes you can alter the style of a button.
Use the action Button > 'Set CSS Style'. I did not use CSS myself yet.
The codes this action needs you can find when you google on "CSS style text".
Option 2/ Use sprites.
There are a lot of technics to do this with sprites.
Let me suggest just one. All GUI buttons go in 1 container. Create one of them to created them all when needed. Destroy one to destroy them all when they are no more needed. Choose one of those buttons to reference to. Lets say that button is 'TheOkButton'.
Now the zoom you gots to cage with ...
System > Compare two values > TheOkButton.count = 0. (if there are no TheOkButton sprites on the screen, the zoom can work.
Then all the stuff you have in place to zoom.
No clickable/touchable problems during normal gameplay, they are just not there.