Chrome OS user and Redditor InauspiciousPagan shared a screenshot of a Chrome OS device with a built-in Night Mode option, which seems to be baked into the newest Canary channel build of Chrome OS. The screenshot shows the feature's activation switch alongside the brightness and display settings, and does have the feature turned on, but it apparently is not picked up in the screenshot. The night mode on offer is reportedly going to be included in Chrome OS version 60 as it rolls on down the line. According to InauspiciousPagan, the feature seems to simply swap some color palettes around and make some minor tweaks to make the UI more eye-friendly, rather than actively reducing blue light through a filter like f.lux. Users had apparently been requesting the feature on Google's bug tracker for quite some time.
The device that InauspiciousPagan took the screenshot on was ASUS Chromebook Flip C302CA. Thus far, nobody else seems to be posting up any screenshots indicating that they have the feature available. This could mean that the feature is only available on certain devices in the current Canary build and will trickle down to other devices as it makes its way down the various versions of Chrome OS, though on the same token, nobody has reported being on the latest Canary build and not having the feature, either.
This is the latest in a line of UI improvements hitting Chrome OS as major features, such as Android app compatibility, are slowly worked in and sent out to all sorts of devices. It's worth noting that a proper, screen-tinting night mode can be found in the latest versions of Android, where the Material Design conventions that Chrome OS is slowly moving toward actually originated, being introduced in Android 5.0 (Lollipop). This could mean that the feature will be improved with time, meaning that the current, somewhat inferior iteration is merely a placeholder, or is being tested and experimented with by the Chrome OS team. The Stable channel of Chrome OS is currently on version 58.0.3029.112, and version 60, which this feature is reportedly riding in on, is estimated to be in the Stable channel this August.