Magneto 2 site with Argento theme has an error on checkout shipping page

I had a coder has inadvertently introduced an error on a page on my web site, which shows itself only on Internet Explorer 11. He was (I believe) trying to change the hex colour of the NEXT box on the shipping page during checkout. On other browsers it works fine, but on IE11 there is a : symbol and a weird additional red box (see attached), and when you click next it does not progress to the next page, it just does nothing. I want to replace this faulty file with the original unaltered version, however I don't know what file he changed and the Argento support people can't tell me what file contains the code to generate NEXT box on that page. The weird thing is that we have another web site which is a duplicate of this site and although that page contains the same : symbol and square box, it WILL let you click next to progress to the payments page. So the two sites need the : and box removed on IE11 and the site which cannot progress to the NEXT page needs repairing too.