Your thermostat's display brightness is at maximum when you're using it. When your thermostat is in sleep mode, the display brightness is automatically adjusted based on ambient light by default. You can change your thermostat's display brightness in the Home + Control app: go to settings, select "Manage my home", select the room where the thermostat is installed, select the thermostat and select "Advanced", you can then set the display brightness to always off or set a custom level.