I am not a web developer, nor am I familiar with OnlineGDB's web development intricacies. What you do looks right to me, so maybe the issue is specific to OnlineGDB.
One possible solution is to just open the site on your own PC. Every PC these days has a web browser that can load websites for you (you're looking at one right now).
Another workaround would be to place the CSS inside your HTML file, as described here, in section "Internal CSS".