One of the new cosmetic changes in Android 8.1 is to do with the presentation of the quick settings interface. With the introduction of the final version of Android 8.0 (Oreo), the quick setting interface changed to a white color, often referred to as a ‘light’ theme. It actually was changed much earlier than the final release of Oreo as the lighter theme first showed up for Android back when it was more simply known as ‘Android O’ - with the release of the second developer preview (as shown above). However with the launch of Android 8.1, the color of the quick settings menu is now no longer pre-determined and instead is largely dependent on the color of the wallpaper currently in use. So, for example, if the wallpaper is light colored then the quick settings interface will now show in the white (or light) color/theme, while if the wallpaper is dark colored then the quick settings will show in a black (or dark) color/theme.
Owners of the Google Pixel 2 and Pixel 2 XL will already be familiar with this feature as this is one that is offered to those device owners out of the box. So in reality what Google has done with Android 8.1 is expand the feature out to the first-generation Pixel devices, as well as the later Nexus models. For example, Android 8.1 is now available for both the Nexus 5X and the Nexus 6P, and owners of these Nexus device are also reporting on Reddit that the feature is active after updating. So this is not a feature that is specifically only available to the Pixel line. If a device is capable of installing the Android 8.1 update, then this new light/dark quick settings color feature will be available and activated by default.
In addition to the general light and dark theme based on wallpaper feature, the quick settings background also now appears slightly more transparent than it did before. On the standard version of Android 8.0 the quick settings made use of a more solid color. Now though, and again regardless of whether it is the light or dark theme in use, elements behind the quick settings interface will remain somewhat visible - as shown below.