Tested on AOSP Extended ROM and Lineage OS 15.1.
Make sure your system supports the use of a Substratum theme. Download the Substratum theme engine app and the theme. When you open up the Substratum app you’ll find the theme inside.
The absolute best way to keep the theme working over time: Every time you update an app to a new version from Play Store, there’s a risk that it don’t match the version we based the theme overlay on.