You can use either online version of the client, or standalone version for Mac, PC or Linux. All versions support automatic update! Also downloads are available from the official site - https://wavesplatform.com/product
Choose and open your Waves wallet. Click on the Settings button (see left bottom corner)
You will be forwarded to the following settings page.
Choose General and select Dark from the drop-down menu in the Theme section.
After a few seconds, you'll see the dark interface.