Hello,
Have you removed custom.css file?
There is no one, just default.custom.css.
Please ask your host to restore that file from the back up.
Did you make back up yourself? If so you can restore the file from your archive.
Also I’ve checked “Add to Cart” button and products are added to cart successfully.
Please clear your browser cache and check again.
Thank you.
Regards,
Eva Kemp.