Microsoft recently introduce new their new field service mobile app and planned to retire their Resco based mobile app on June 2022. In new Field Service mobile app has set of limitations and documented them in following Microsoft document
https://docs.microsoft.com/en-us/dynamics365/mobile-app/mobile-offline-capabilities
According to the documentation CSS are not fully supported in offline mode.
How to fix: You have to add all CSS code in line with HTML to render CSS correctly in mobile offline mode.
