I have just upgraded all of our projects to 2019 Classic from 2015. All were publishing fine as standard WebHelp in 2015. After upgrade, a lot of CSS files are not being included in the publish. The pattern appears to be any CSS filename which has spaces within it and ends in a numeric does not now get included in the publish.
Any ideas on how to fix this problem? Have tried applying recently released update 11 but didn't make any difference.
I wasn't aware of Adobe having made any changes in that respect but see this page re general naming conventions for CSS files.
At the risk of stating the obvious, renaming the files without spaces or numbers would fix it. In Topic List you can select all the topics, right click and select Properties. Then you can apply the newly named CSS to all the topics in one go. That should fix things in a few minutes.