assuming j7 prime is running on Android oreo 8.1 there is no dark mode available in that particular version but no worries you can enable dark mode by this small app basically it mimics the dark mode functionality https://play.google.com/store/apps/details?id=systems.maju.darkmode
And one more thing after installing this app ...make sure you open samsung email and go to settings and turn on night mode there as well so that dark mode doesn't crash ..
