There is icon for theater (comedy tragedy) on Apple Watch and face is always black. How can I delete icon?

Theater Mode can be turned on OR turned off. It cannot be deleted entirely as it is baked into WatchOS and is necessary for the situations it was designed for.

To access turning on or off, swipe up from the bottom of your watch screen to open Control Center. See Theater Mode. Turn on or off as needed.

 Use theater mode on Apple Watch.

You can customize the control Center that has the theatre icon

Remove Control Center buttons

You can remove the buttons in Control Center by following these steps:

1. Touch and hold the bottom of the screen, then swipe up to open Control Center.
2. Scroll to the bottom of Control Center, then tap Edit.
3. Tap  in the corner of the button you want to remove.
4. Tap Done when you’re finished.

To restore a button you’ve removed, open Control Center, tap Edit, then tap  in the corner of the button you want to restore. Tap Done when you’re finished.

Use Control Center on Apple Watch - Apple Support

To add some context, Theater Mode when turned on will stop your watch face from lighting up while you are in a movie theater, or any situation where you don't want the watch face to light up or turn on.