Seems indeed an issue with pretty much all themes.
Another issue. The box-shadow, applied to views outside of the active one, is far from smooth and somewhat distracting. Which invalidates goal (3).
See printscreen below. Not sure if it is just me, present on both my screens (ultrawide screen and 1080p 15inch laptop screen).
Apparently layering should smooth things out. Below css is the best I could get:
div.workspace-leaf.mod-active {
box-shadow: 0 0 10rem rgba(0,0,0,0.01),
0 0 20rem rgba(0,0,0,0.02),
0 0 40rem rgba(0,0,0,0.03),
0 0 70rem rgba(0,0,0,0.04),
0 0 100rem rgba(0,0,0,0.05)
;}