This extension is just an unofficial support that allows you to enable the dark mode theme on Whatsapp Web.
WhatsApp Web has an official dark mode support, but for some reason the developers of WhatsApp decided not to add it as an official feature yet. This most likely is because they still are working on finalizing the changes. Use this extension wisely as it could break depending on how WhatsApp developers proceed.
All the dark mode support credits goes to WhatsApp team. I do not affiliate with the WhatsApp developers and this extension is made for your personal use.
Feel free to make suggestions and report bugs on the github page of the extension!