Hi Todd,
we usually recommend the file to be stored in the same server as the Moodle site to have everything in the same place. In the case of Moodle cloud, you can use any server or service that allows you to publish public files.
For example, in one of our testing sites we use a gist in GitHub, so the URL is something like:
https://gist.githubusercontent.com/user/.../moodlemobileapp.css
I think you should be able to use some services like Dropbox or Google Drive as long as the file is public and you provide the download link in there. Please notice there shouldn't be any "intermediary" page when opening the link, it should be a direct link to the file.
In 2016 I did a presentation about customizing styles in the MoodleMoot US, here are the slides:
https://www.slideshare.net/DaniPalouSala/creating-moodle-mobile-remote-themes-moodle-moot-us-2016
Hopefully they will help you
Kind regards,
Dani