When you open the theme in the substrate application, the drop-down menu appears above the layers. You must choose one based on your version of Android ROM. There are also overlays with different variants for a specific version of Android.
AOSP applications such as Clock, Messaging, Gallery, Contacts and Dialer may or may not work because each custom ROM has its own variant of these applications.
Supported Android
{7.0 and UP}
Supported Android Version:-
Nougat (7.0 – 7.1.1) – Oreo (8.0-8.1)
Download Mono Art substratum 26.3 Patched APK For Android