Google chrome (or) Mozilla Firefox with the latest version must be installed in your DESKTOP.
Once the WhatsApp Web client is open, right click on the empty area outside of the chats, and click on the ‘inspect page’ tab. If you cannot see an empty area around your WhatsApp browser client, use Ctrl + ‘-‘ (minus) to zoom out.
Once you click on ‘inspect’, a splitscreen view will open up to show you the coding elements of the WhatsApp Web page. On this section, towards the very top of the chart, find the line that reads body class=“web”.
Double-click on this line to switch on the cursor, and change ‘web’ to ‘web dark’. Now, the line of code here should look like body class=“web dark”.