You need Greyscale Substratum theme in order to use this app
This is the Accents plugin for Greyscale Substratum theme. Install this only if you are a user of Greyscale.
Find the full theme here : https://play.google.com/store/apps/details?id=substratum.sreerag.greyscale
Note : This is not a full theme and won't theme your device alone. This is a companion app