It's alright if you don't notice the change; we won't mind. :) I'm thrilled to see these kind of small improvements to the admin's visual design, because it's a perfect example of the way WordPress itself has grown: rarely in big leaps, mostly in small, sometimes imperceptible changes. But when you take a longer view, it's quite clear how the changes add up.

Developers who create plugins or build sites for clients should especially take note of the new design handbook. WordPress is really advancing with regard to how seriously it takes its brand and its user interface, and plugins that don't do the same are going to age rapidly in comparison.