I re-install os to Centos8 and fixed this issue
Where to put custom CSS file..
Put the link tag in the custom header.
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome-animation/0.0.8/font-awesome-animation.min.css">
Assuming you're not making your own plugin, in which case you would link to the file in your config less property.