Chrome OS is a niche OS with a niche market. That said, it's been wearing its "niche hat" quite proudly so far.
But as more and more people switch to Chrome OS, as evident by the number of people buying it on Amazon, Google has to accommodate a wider and wider audience.
At the same time, Chrome OS is becoming increasingly popular in the enterprise environment, so maybe Google figured it was time for a more "serious" Chrome OS.
This trend has been long in the making, but some of the more recent graphic changes, spotted by Chrome OS enthusiast François Beaufort, are perhaps the best example of this.
The graphics being replaced are for things like removing external storage or switching on the "dev" mode. You can check out the old version and the replacements in the gallery below.