Android Oreo brought support for themes based on Overlay manager service (OMS).The developers of substratum released Andromeda, A paid add-on package for substratum that allows system wide theming even on rootless devices.Andromeda requires the Substratum application and a one-click desktop too, which allows Substratum to get elevated privileges.

 Andromeda is an ADD-on package for Substratum.By Andromeda That Allow user to install Rootless theme on Oreo. for rootless user Andromeda open's A World of Possibilities. You can apply System-dark themes,change you phone's Navigation Bar color and icon,and lot more

 Right now Andromeda is on sale from Wednesday to new year , Andromeda is sale on $1.20, regular price is 1.99$

 here's the link to andromeda
https://play.google.com/store/apps/details?id=projekt.andromeda&hl=en