Hi Srichandar,
In this article (https://docs.mendix.com/howto/mobile/how-to-use-native-styling) there's a clear guide on how to style your Native app (since you're using Mendix 8.4.2. i assume you're using a Native app). Also this (https://docs.mendix.com/refguide/native-styling-refguide) article could be really useful.
I hope this helps you.
With kind regards,
Bryan de Wit