Make sure you add the UI-resources module in the correct order in the App settings:
This determines the order in which your css is loaded in, if your UImodule is not in the correct position it will get overwritten by ‘later’ styling
Did you upgrade from Mendix 8 to 9?
I ran into the same problem only after discovering that the buildin compiler wont work when upgraded. You can use Calypso to make it compile.
This doc will guide you through https://docs.mendix.com/refguide/moving-from-atlas-2-to-3/