The most visible changes are the background of the "New posts! Touch to see them" toast which is now grey, instead of the previous black, and the Send icons in posts which have also turned grey from green.
As always, if the update hasn't been pushed to your device yet, you can head over to APKMirror and sideload the app (following our handy tutorial).
Other than that, there doesn't seem to be any other noteworthy changes, but hey, look on the bright side, the next update will presumably be 6.0, so there's bound to be some goodies on the way; maybe Google's holding off on any big changes until the update to version 6...
Just hang in there, folks!