A developer from Automattic asks the community for icon themes:

Right now I’m using Genericons with one custom icon, but it might be nice to have something stylistically different this year. We have a lot of flexibility here. Only requirement is that the icon set uses a GPL-compatible open source license. (Melchoyce visited 14.9. 2017)

The look and feel of the theme will change as suggestions tick in.

One of them is this: feather.