Use one file functions.php

Hi! Good plugin, thaks, I use it in my site. Please, dear, ask me, I created theme in /plugins/wp-mobile-detector/themes/, that is, a separate mobile theme, but for some reason even though this theme is its functions.php, he will not react, but instead reads functions typ of threads full version of the site? how to fix it? Thank you)

https://www.ads-software.com/plugins/wp-mobile-detector/