The public release of OS X Yosemite rolled out three weeks ago, and since then Apple has been gradually bringing all of its own services in line with the look and feel of its new operating system.
Having previously tweaked the iTunes Store and its iWork suite, Apple is now updating the Mac App Store, adding the thinner fonts, simple white backdrop and gray separators synonymous with Yosemite.
As of now, only some tabs feature the newer design, while not everyone is seeing the redesign. Some users have reported not seeing it at all, others are seeing it intermittently, and yet others permanently. You can launch the Mac App Store from Yosemite to see if you currently reflect the update.