This theme won't be updated anymore. Support ends with Pie. Rounded UI Pie/Oreo/Samsung/Oxygen is a substratum theme. If you don't know what I'm talking about, please don't download this theme because you won't be able to use it. This theme supports: Android Pie AOSP stock rom and custom roms. Android Oreo AOSP stock rom and custom roms. Samsung Oreo 8.
It's secure to download the app. Gianluca Spadazzi is the official developer of RoundedUI Pie,Samsung,Oreo,Oxygen Substratum Theme which can be found in the Personalization category. Google Play shows that this application is rated 4.2 by its users.
Over 1K users downloaded and set up the app and its most recent release was on Mar 24, 2019. It's free to download the installation package of RoundedUI Pie,Samsung,Oreo,Oxygen Substratum Theme, also, you can take a look at users' reviews on Droid Informer.