This is a Dark Version of the Twenty-One Client Theme that is included with WHMCS.
It is perfect for those websites using a darker design and want a client area that is also dark. It is 100% compatible with the latest version and works as a child theme to the twenty-one style so it is easy to keep updated.
It also includes a dark version of the Standard Cart orderform.
Check out the images in the Marketplace:
https://marketplace.whmcs.com/product/6194-twenty-one-dark-theme