To activate Dark Mode, click on the three vertical dots next to your profile picture and select “Settings”. There you will see the entry “Design”, which you must click next. You then have the choice between “light” and “dark”. The latter is the new dark mode.
If the “Deisgn” item is still missing in your settings, it often helps to simply reload WhatsApp Web. The dark background is currently distributed to all users, so you should be able to find the setting in good time.