My 2cents but I have noticed lots of small, minor updates to their front end over the last year. Things like better focus and hover states, branch copying, better mobile layouts, etc. Lots of minor front end polish.
That was an explicit paper cuts project by the GitHub team started prior to the acquisition.
Primary push was the Dear GitHub letter and the isaacs/GitHub repo which documented hundreds of such small issues. Also, the GitHub refined plugin, features of which GitHub keeps reimplementing now.
