The error "The customized CSS file could not be found" is presented when a site is using a custom CSS and that Lesson is imported to a new site without bringing over the original css/resources.
Can the default css be applied and this error message not be shown if there is no custom css available?
Steps to replicate:
1) Create SiteA and add Lessons and Resources
2) Add custom CSS to Lessons in SiteA
3) Create SiteB and add Lessons but not Resources
4) Go to SiteB -> Site Info and import from Site -> Lessons