In kwrite > settings > color themes > colors
I see that one can change the underline color of a spelling mistake line
but how can I apply a background color to a spelling mistake?
I see lotsa background color options... but I'm missing this one...

I've searched... and it appears there isn't that option
however I did find an elegant and simple solution...
I changed the underline color of a spelling mistake line to the same color as normal text
in Breeze Dark, that default red color is somewhat obscured
but now a spelling error (obviously) pops out!