It's my understanding that your screen is too dark when watching movies on Netflix. I have some suggestions that may help. Are you using an app or are you using a web browser? If using the web browser, try a different one and see if the issue persists.
Another alternative is you can adjust your display brightness: macOS Sierra: Adjust your display’s brightness or you can go to Apple Menu > System Preferences > Displays > Color and adjust the display profile to see if that helps. If this is specific to the app you may want to reach out to the developer.