The Night button changes Orbitrack's appearance to red-on-black, to help preserve your night vision. Night Vision is best used in the dark, out under the stars. Tap this button once to switch to Night Vision; tap again to restore Orbitrack to its normal appearance.
For many people, the screen is still too bright to effectively preserve their night vision - even when Orbitrack is using Night Vision.
Your device system settings may let you adjust the screen brightness. To turn down the screen brightness for all apps on your devices, use the system screen brightness silder.
An even better solution is to place a piece of red film over the screen. A brand called Rubylith seems to work particularly well for this purpose. A "hardware" approach to preserving your night vision works better than any software solution because it will enforce a red appearance across all applications, not just Orbitrack.