In Safari Preferences > Advanced screen, there is an option for Style Sheet. Looks like you can create a customized Safari page, and make that the default. I've never tried this, and don't know for sure if that's what it does. It might be worth searching on Apple Support to see if you can find out what this is for, and, if it's what you want, how to create your own. Maybe someone else knows about this and can enlighten us.
Your screen shot looks like Theater Mode, which is available on some web sites, such as YouTube.