The dark theme will arrive for WhatsApp, we just don't know when it will happen. Who uses WhatsApp Web, however, can count on some tricks to receive this feature in your browser. You will need the Chrome browser for this.
How to use the dark theme on WhatsApp Web
There are two options that enable the dark theme for WhatsApp Web. The first one is a Chrome command, which you will find by accessing its advanced settings. In addition to changing the color of WhatsApp, this option brings dark Mode to all the sites you access.
First, leave WhatsApp open in Chrome. Then, open a new tab in the browser, paste the address chrome: // flags / # enable-force-dark and click "enter". The option you will use appears at the top of the list in yellow "Force Dark Mode for Web Contents", open the dialog box and select "Enabled with selective inversion of non-image elements ''. This option causes only texts and backgrounds to be inverted, except images.
Restart Chrome to apply the changes. Your WhatsApp Web looks like this: