Cantarell is the standard UI font for GNOME. I made several enhancements to it in the
past.. 2 years I think and redesigned it for 3.28, so you'll see it on e.g.
. It got pulled from Fedora 28 though due
to missing Cyrillics I think, which I'm still working on.
You can build the fonts yourself if you want to have a look:
Cyrillics WIP: https://github.com/madig/cantarell-fonts/tree/cyrillics