Earlier today Android 8.1 (Oreo) became available as part of Google's Developer Preview program. So while the final release of Android 8.1 is not expected to become available until later in December, those interested in getting to grips with the newest version of Android (and providing they have a compatible device) can download the latest developer preview version now. Speaking of which, this one does come with a few changes and one of the most noticeable after first installing the update is to do with the navigation bar.
It seems Android 8.1 has introduced a design change which sees the buttons in the navigation bar dimming after a short period of time. So while at first they do appear in their full and complete form, shortly afterwards they begin to fade and blend in more with the background. At present, this seems to be in effect with all apps and also on the home screen. Adding to this fading button change, in settings, the whole navigation bar color has changed from black to white which seems to make the settings pages in general appear more harmonious due to the white colored theme in effect in settings.
So regardless of which app is opened or whether or not the navigation bar appears in white, black or transparent (like it does on the home screen), the same fading effect with the buttons in in place. While in reality there is not much of a functional benefit to having this feature in place, it could be the case that Google is looking to lessen the load on the display caused by the brightness of the navigation buttons and especially considering how these buttons are one of the constant aspects in use when navigating Oreo's interface, irrespective of whether the user is in an app or not. So toning down the impact of these buttons may prove to cause less of an issue with the display in the long term. You can see an example of how the dimming effect looks in the image below (provided by Android Police) which shows the light gray colored buttons against the black backdrop of the Google Play Store's navigation bar.