Conditional Commenting CSS for different browsers

Hello friendly helpers!
I am using a free wordpress theme called Viewport here: https://www.too-legitimate.com
I got the theme here: https://paulicio.us/2008/11/05/viewport-a-free-wordpress-theme/
In Safari the left/right buttons scroll correctly, however in Firefox and IE (I haven’t been able to check others) they do not function. I understand I can use a CSS Conditional Comment (learned here: https://quirm.net/2009/06/29/wordpress-conditional-comment-css/) to tell Viewport to act differently when used by other browsers. My problem is that I don’t know specifically what is causing the problem. I have tried changing the size of my header, to no avail. If anyone has an idea of what to change I’d be so thankful!
Brian