Greasy Fork is available in English.
Adds a button to toggle a dark youtube style.
< Părere la script-ul Dark Tube
Sorry for the late reply.
I'm not sure what you mean by permanent. Once you set it to dark the theme should remain like that until you change it back.
Request
I wanted to request if you could make it so the theme is permanent or add an option for it and add an option to hide the button if one decides to select the option to make it permanent.