Found a bug while testing in w3c Validator

When I was checking a site created with help of CBB, I was testing the site on https://validator.w3.org/ and found that it shows some CSS error. at some places there are class selector coming inside the values. For example somthing like {overflow:hidden; .md-c ….} . Here I have attached the screenshot of error too. https://prnt.sc/sJrdP0QKF22n , Please have a look.