The problem is the border. I’m sure that’s clear by now. It’s got nothing to do with accessibility settings.
My question was directed at Pavel, who was saying the border is black, which isn’t expected behaviour. If the border is coming up black on a light background that is a problem that should be fixed, and it would be nice to know what settings are necessary to see it, whether they be in Scrivener or outside of it.
The only way I know of to get it down to black is to set the highlight itself very dark—like it is in default Composition mode settings.