Yes, it is indeed hard to see, especially as it appears to be overlaid on the cell's normal black border.
The following just as general information to make my case (and show-off
) in case it is easy to change the highlighting:
I have some experience here in another field
. As BR may know from boating days a number of underwater offshore installations such as pipelines and submarine cables are shown on charts in magenta; well supposedly magenta as it depends a bit on ones monitor or the temperature of the light if using a paper chart. Some of these features, especially submarine cables which are a thin wavy line, can be easily missed (just like the Calibre cell border highlight) if the line is thin and pale, as it can be on some raster charts.
How red (magenta) came to be used for those charting purposes I don't know but possibly it was decided before it was well known that the sensitivity of human eye photoreceptors is very low for red and so a lot of red is needed for good visibility (green-yellow e.g. like lime, is said to be the best for emergency vehicles and is used here in NZ to some extent or another for ambulances, fire engines and police vehicles; also for high visibility vests). Furthermore, as light levels decrease, red turns to black.
So, it seems to me that two things come out of that with respect to the spell checker cell highlighting. If red is to be used it needs to be bold; so thin and dotted lines should be avoided. And it should not be overlaid over black as in lower light conditions the red will appear as just being part of the black line and so disappear.
The latter point seems to me to be so even with backlit displays (e.g. LCD) as, for example, with the Calibre dotted cell highlighting I can make it turn towards black just by throwing a heavy shadow over that part of the monitor in an otherwise day-lit room (but probably not so with emissive displays e.g. AMOLED).