The Phantom app doesn't have a brightness setting. If you want to change how bright the app looks, use your device’s display settings.
Mobile app
Phantom uses your phone’s current brightness level. You can adjust it in your device’s display settings.
When you tap the + button in the lower right, then tap Receive to display a receive QR code, your device may automatically increase brightness. This is a system-level feature for better QR scanning. You can still manually lower the brightness using your device controls.
Browser extension
Phantom follows your computer’s display brightness. To adjust it, change the brightness in your system settings or on your monitor.