Can't change Grid selection checkbox checked background fill color.

Grid selection checkbox background color seems to be hardwired to blue without a variable to control it. In the valo theme examples it never changes. Go to link below, click a them like "Flat dark" and go to Grid/Tables. it is still blue when the rest of the theme has change to a dark organge/white/gray color pallete. I've attached a screen shot.

Is there a way to change this color with a $<variable>?

https://vaadin.com/valo