Finally found correct info on what styles needed changing.
Don’t know how to remove the U/L but I now have good strong backgrounds to errors
Parse errors in code are not marked in a way that is visible to me.
It seems that syntax theme and particular language (CSS, JS, JSX) do not affect this.
dev mode is not showing me a class on the element that seems to be the parse error.
In fact, it seems that the underline is not even part of the text being underlined!
I need to highlight using other than the thin red dotted underline.
How is this highlight controlled? Seems to not be like theme colors.