Welcome to Android Central! If you're talking about Android Messages, they took that option away with the latest update. It's been causing a bit of an outcry: https://www.droid-life.com/2018/09/...the-new-android-messages-even-with-dark-mode/. The best thing I can suggest is to send Google feedback through the Android Messages app's Help & Feedback menu option. If they get inundated with enough complaints about it, they may very well bring that option back with a future update.