Funny enough, Pencil Project in itself is a testament to how badly open source software needs better/more designers who know good UX/UI. Try putting a modern designers coming from Figma/Sketch in front of Pencil Project and watch them puke colorless rainbows while trying to figure out how to do the most basic of operations.

Figma, because of the cost (or lack off) and availability, seems to be the only tool realistically available for doing open source design today. Unfortunately neither the format nor the editor is open source itself, but sometimes the means justify the end I guess.