It was designed by Ann Magill and originally looked a little bolder as Magill had an association with the Jolly Roger flag in mind, but it turned out not to be accessible for a lot of people with for example visual or neurological disabilities. So together with people from the community the above came about. I'll link you Magill's reddit post, but here's a short breakdown by your resident disabled:
The stripes each represent a group of disabilities and the load of black/charcoal is for /3