The hybrid app loads CSS references differently than on desktop (and possibly from how it was done with Mendix 6… I’m not 100% certain). In your project’s theme folder, there should be a file called components.json. The hybrid app uses this file to know which CSS files to load. If you have a custom file outside of the normal project theme, you should add it to the list of css files in this components.json file.
Reference document: https://docs.mendix.com/refguide/customizing-hybrid-mobile-apps
In Mx7 you can create the login screens from within Studio Pro so you would not need the custom login screen anymore. Did you already tried that?
Regards,
Ronald