You will be required to install the Substratum Theme Engine and have root access OR the Andromeda add-on, if you want to apply this theme: For rootless theming with Substratum, you will have to purchase the Andromeda add-on pack: A bare-bones Black theme for Android Oreo that focuses on nothing more than theming the system.
Substratum Themers Collective is the official developer of this application that can be found in the Personalization category. The latest update of Sai's Oreo Theme was released on Mar 5, 2018 and the app was downloaded by over 100K users. Google Play shows that this application is rated 3.1 by its users.
Download the installer of Sai's Oreo Theme 5.0 free of charge and take a look at users' reviews on Droid Informer. The app can be launched on Android devices 8.0 and higher. The app is totally secure to download.