Help Center 2.0 — scheduled for release May 11, 2021 — does not support CSS customization. Instead, it features built-in customization tools.
However, we continue to support your use of CSS-based customization until you upgrade from Help Center 1.0 to Help Center 2.0.
Have you added any custom CSS to your web support portal? If so, please check whether you added the ‘overflow: hidden;’ property to the body tag. This tag prevents the page from scrolling, and removing should resolve the issue.
To learn more about customizing your CSS, see How do I customize my web support portal?