This is a CSS based theme for enabling Dark theme on WhatsApp web. the repo includes files for Chrome Extension

• Open Chrome's Extensions page chrome://extensions
• Drag and drop .crx file on extensions page

  OR

• Clone the repo git clone https://github.com/lakshaysinghal/WhatsApp-Dark-Theme.git
• Open Chrome's Extensions page chrome://extensions
• Enable Developer Mode
• Click on Load unpacked and select the Whatsapp-dark-mode folder
• You can also pack extension and drag and drop the .crx file in the extensions page
• Open WhatsApp Web and login

• Copy the content of Bookamrklet file
• Right click on the Bookmark Menu and select new Bookamrk|Page
• Paste the code in Location input field and give it a name
• Open WhatsApp Web and login
• Click on the bookmark created.

There are still few caveats left to fix. If you add/resolve those send a pull request.

Being CSS based, if WhatsApp devs decide to change anything on UI front this code might fail.