jquery-ui-rails-5.0.5, sass-rails-5.0.3
jquery-ui-rails-5.0+ changed the naming convention for its assets. Instead of jquery.ui.(whatever), it is now jquery-ui/(whatever). The Stylesheets are named (whatever).css. This causes the highly-recommended SASS @import option to fail, while CSS require continues to work.
I have been working on this problem this morning and I tracked down an answer in SASS-RAILS that was said to work, but didn't. It was to include both core and theme with the desired module as follows:
@import 'jquery-ui/core';
@import 'jquery-ui/datepicker';
@import 'jquery-ui/theme';
I wanted to post the answer that I discovered here, in case it helps.