In reply to Daron.

@Daron, all they mean is a font that's installed on the device accessing the site.

So, because they list Roboto as one the fonts and I have it installed on my PC, that's the font showing on the admin pages, as if I were using my Android phone!

I actually use Linux Mint as my OS, so have the Ubuntu font installed, but that doesn't show up because Roboto is listed ahead of it. As I said, bizarre!

So, yes, I have tweaked this so that the only font specified is sans-serif. But tweaking it like is no better than the tweak I had to apply before to remove Open Sans.