Hathor admin template has not been updated to work with the new features
We strongly recommend all users to use the Isis admin template. The Hathor admin template has not been updated to work with the new features shipping with this release. When we released Hathor it was built with accessibility in mind, but over the years the Isis template has been improved in terms of accessibility and supporting technologies. We now believe that the Isis template is as good as or possibly better than Hathor.
The Hathor template will be removed from the distribution with the release of Joomla! 4.0.
No errors have been reported, however new features may not be available when using the Hathor template (e.g. the customisable admin menu) or layouts may not display correctly (Hathor requires layout overrides for all views otherwise the default layouts, which are designed for the Isis template, will be used instead).
This pertains only to Joomla! version(s): 3.7.0
What is the cause
See GitHub for additional discussion: https://github.com/joomla/joomla-cms/issues/15079
How to fix