This will happen if there are missing asset files in the core/assets folder and/or the custom/templates/DefaultRevamp folder - you can confirm this by checking your browser console (typically opened by pressing F12).
You can try reuploading both of these folders from the latest release to ensure all files are present.
Can you please download the latest nameless-deps-dist.zip release file again: https://github.com/NamelessMC/Nameless/releases/tag/v2.1.2
Version 2.1.2 is now available!
This is another minor update with several fixes for your website, including a potential error during user registration, broken user CSS on the members list and the forum index showing topics without having the "view other topics" permission.
Existing websites should update through the StaffCP -> Update tab, and new installations can download nameless-deps-dist from the release on GitHub.
Thanks again to the whole team and all contributors for making this release possible!
Please visit yoursite.com/index.php?route=/panel/upgrade to run migrations
A fix should be available in the next release which will hopefully be available in the near future.