I don't know if that is already a known issue, but I personally don't like the font-family used in the LayerList widget, after the update of WAB 2.1 (online and dev).
So (for dev version only) I searched a bit in the CSS's and found some font-family definitions like Avenir-Medium, instead of Avenir Medium.
Basically I replaced all the appearances of that "-" in the following files, which fixes it.
- \LayerList\css\style.css (Lines 4, 30, 459, 493)
- \jimu.js\css\jimu.css (Lines 3597, 3746, 3776) (affects other widgets, e.g. “Robert’s eSearch Widget”)
I also informed the Esri Support Team about it.
PS: Just got a note from support that they already logged a bug: BUG-000097347: The font used for items in the Layer List widget doe..