Can you check if you followed the documentation https://docs.moodle.org/35/en/Moodle_app_guide_for_admins#Customise_your_app_theme?
If you followed it, and it is still not working as expected please indicate us version of the Moodle App as well as device and its Operative System and version (e.g. Android 8, iOS 12, Windows 10, ...) are you using.
Thank for the reply.
I have an apology to make, I had assumed that this would also work on the demo (https://mobileapp.moodledemo.net/) which I thought was just a web version of the app.
I assume this is not the case as my styles do not take affect, they however have taken affect when checking an android device.
in order for the styles to work in the webapp (mobileapp.moodledemo.net) you need to disable CORS in your browser: