Calendar app - make day divisions more pronounced?

Whilst I love NextCloud and have been using for ages, in my shared hosting, I find that the calendar app ‘grid lines’ for month view a little faint. Particularly on a busy calendar, which mine is.
Some days don’t even show an outline if a multi-day event ‘passes through’.
How would I go about ‘thickening’ these lines? CSS somewhere I guess.
Hopefully compatible with app and nextcloud upgrades although, this is less important than thickening the lines is.