It's probably already answered, but I'm just not using the right words in my search to find it. Sorry for any redundancies I'm introducing.

When I hover my mouse over an event I get a box that has very dark grey text over a black background ... not quite impossible to read, but a very close second.

I haven't found a place that allows me to control those colors/contrast, but would love to. As is, the text boxes are useless.

These colours should be specified by the currently active colour scheme on your system.

Colour schemes can be managed in System Settings > Application Appearance > Colours.

System colors was certainly the right path to saunter down...
It took several tries with Ubuntu, but
System-preferences-appearance-theme-customize-colours-windows-text
got it without having to reboot (again).

Thank you for pointing me in the right direction.