This supports Android Oreo without root, using Andromeda! If you have root, everything should work out of the box except on Samsung!
https://play.google.com/store/apps/details?id=projekt.andromeda


Support chat:
https://t.me/substratum (#support)

Take over control of the finest aspects of the system UI and personalise your Android experience without limits with the official Substratum theme engine, free to use and powered by the constantly growing, community-driven collection of beautiful overlays.

Attention:
Google is preventing stock OMS users from utilising custom overlays. To help us bring back rootless theming support to Android, please star the following issue in Google’s Issue Tracker: https://issuetracker.google.com/issues/74354703.

Theme engine support:
• Stock Oreo (OMS): Compatible with rooted and unrooted devices, in the case of the latter the Andromeda plugin is needed for Substratum to function correctly: https://play.google.com/store/apps/details?id=projekt.andromeda

Community links:
• Telegram: https://telegram.me/substratum
• XDA: https://forum.xda-developers.com/apps/substratum/
• Reddit: https://www.reddit.com/r/Substratum/
• CrowdIn: http://translate.projektsubstratum.com/
• GitHub: https://www.github.com/substratum
• JIRA (Bug Reports): https://substratum.atlassian.net/servicedesk/customer/portal/1

Detailed guide:
https://forum.xda-developers.com/apps/substratum/official-starting-substratum-andromeda-t3798316/post76687469#post76687469

Disclaimer:
Having external, third party theme clients installed may cause issues with Substratum, including unforeseen bootloops, crashes, and more. Customer support will refuse to provide assistance to users running such applications, including Swift Installer.