You can go through this link: https://www.clevr.com/blog/add-dark-theme-mendix-app.
Hope this will help you.
This is the toggle in Firefox:
*Editted* As for the youtube movie you mentioned, what version were they showing? Probably a near future feature :-) :
Hi, thanks for the answers. I was asking because I saw that the dark mode was already integrated in the new update, but I can't find the option to activate it.