Echo JS 0.11.0

It makes buttons, tags and labels less rounded. Fixes terrible green color of primary buttons. It makes all buttons look like buttons (less flat)

I think I guessed it right then. It doesn't "fix" anything, as everything works fine. It appeals your own sense of aesthetics.
For this kind of things, I suggest creating a use style sheet to be installed with Stylus or other similar extensions. GitHub's new UI is... fairly new, so it's subject to change. Updating a Chrome extension can be a hassle.