Hi,
You have done all the right things, however, the problem is in the Moodle Mobile App itself as you need to get the CSS ID & Class Selectors from the App so that you can make changes to colours etc.
I do not think the Developers of the App envisaged the problems that the ordiinarry person doing these changes without good documentation, If you search the Moodle Docs you will find one or two that kind of talk about this but don't give you that much help.
Here is a link to the question I asked in the Moodle Mobile Forum,
https://moodle.org/mod/forum/discuss.php?d=356166
You may be better to ask a question there and copy the link for your question here and post that link in your post in that forum, then you are not duplicating posts.
Hope this helps?
Mary