Costumizing theme error!

So when I’m trying to live preview a theme and costumize it, I am able to view the theme, see the costumization menu, but whenever I’m trying to click on like “Colours” or any other option in that menu, nothing happens. I cannot costumize any of my themes!
I’ve never had this problem before, it started accuring a few days ago..

I have no idea what’s wrong… Help anyone?