Dark Mode is finally coming to Chrome OS soon. The corresponding feature has already appeared in versions in the experimental channel Canary.
Accordingly, in order to get access to dark mode in Chrome OS right now, you will need to install an operating system build from this channel. It can only be accessed from Chromebooks that are in dedicated developer mode. However, Google warns that such assemblies may be unstable. After installing Chrome OS from the Canary channel, you need to open Chrome and enter the text chrome: // flags / # enable-force-dark and chrome: // flags / # enable-webui-dark-mode in the address bar.
So far, Chrome OS dark mode is experimental and buggy. However, it applies to the entire user interface, not just activating a darker background.
It is not yet specified when dark mode will reach stable builds of Chrome OS and will be available to all users.
Source: The verge