thanks for trying out the beta of Mendix 5! From beta 8 to our final release, we integrated bootstrap 3 support that changes the way some elements are rendered. Besides, we changed certain naming conventions and display properties in the stylesheet to improve on our overall consistency in the way things are handled on the front end.
We have outlined a document that summarises some of the most important changes that make the theme more consistent with the rest of the front end elements. Older stylesheets will have the older naming, for instance, which makes it break - which is only an issue for our beta users, of course, as anyone starting from scratch in our final release should be fine.
I’ve dug out your email from our support systems and have sent you a document that outlines some of the impactful changes beta users might run into. If you apply some of the handful of changes back to your theme, you should be a good amount closer to what you had before.
If anyone reading this visits the forum with a similar question coming from our beta release, feel free to email me at firstname.lastname@example.org and I’ll gladly send you the same document.
I've ran into the same issue, having a custom css file for the project theming and then realising that the names for the classes had changed. Unfortunately, I could not find sufficient documentation for an elegant or even effective way for "migrating" themes, so I rebuild the css file with the new names for the classes.
That being said, i wish you good luck and maybe someone with a bit more insight can provide a more helpful answer.
I've sent you the same document as well. Do email me any feedback on what items or explanation may be missing, so I might be able to help out if things still are in too bad of a state for you to continue implementing your theme.