This is how to convert WhatsApp web to dark mode
If you also want to change the background of WhatsApp web to dark mode, first open the web version on your computer or laptop. After that go to the mobile app. As soon as you open the app, you will see three dots on the top right corner. After clicking on these dot, the option of WhatsApp web will appear, which has to be tapped. Click on it to scan the QR code on the screen. Your account will be logged in after the scan process is complete.

After logging in, go to WhatsApp background and right click. Here you will see a box. In this, you have to click on the Inspect option.

As soon as you click on the 'Inspect' option, a console will open on your screen. Now you have to go to this console and click on the body option.
After doing this, instead of web in the body string, enter dark and press enter. After pressing the enter button the background of your WhatsApp web will go completely dark. However, as soon as you refresh or reload the page, the WhatsApp web background will turn white as before.