Google Android 7.0 Nougat comes with Lots of Amazing features, Most of them are same as that of Preview Builds of Android N, But Nougat seems missing The "Night Mode" feature.
From The Release of Android Marshmallow, We were expecting "Night Mode" in Next Release of Android. But Google Pulled it back again, Though The Feature is hidden in Android 7.0 Nougat and You can now enable it using an Application called "Night Mode Enabler".
Night Mode gives a Darker UI to your Smartphone. You can Download "Night Mode Enabler" from Play store. Night Mode enabler will help you to tuck Night Mode Option to SystemUI Tuner.
Download- Night Mode Enabler (Play Store)