I found a different way around this. Not being satisfied with the conditional formatting solution (- owing to the failure in recurring appointments not being affected), I tried just changing the calendar colour away from the default. (View>Color). Picking something more garish such as the bright blue, or pink, makes the subtle emphasis on today contrast more starkly with the grey background, so it's more effective.
Now I just have to put up with a fluorescent calendar 😅.
Also, moving my Reading Pane (View>Layout>Reading pane) to the Righthand side and increasing its size, so that the day columns are narrower, makes it easier to differentiate them. (This is more of an issue on a large monitor than a smaller one, i.e. laptop).