I would create a PPA, but I ran into problems compiling the package (regardless of patches) nor did I see the patches in action on Ubuntu MATE 19.04. I put it aside for now, but it works great on Manjaro for the GTK apps I use on KDE.
I have doubts Canonical would take these patches as main Ubuntu intends to create an experience around GNOME 3, in which these patches would break GNOME and other Ubuntu flavours/derivatives. Someone could create an alternate package (or PPA), which is how Manjaro handles it with gtk3
and gtk3-classic
.
I've created a separate topic to continue the discussion of gtk3-mushrooms: