After a brief search on 'thunderbird userchrome calendar current day background color', I arrived at some code that changes the color of the header in Week view to make it more distinctive. It doesn't affect the code for month or multiweek. Further refinements are probably possible.

calendar-event-column[selected="true"],
calendar-header-container[selected="true"] { 
  background: yellow!important;
}

calendar-event-column[relation="today"],
calendar-header-container[relation="today"] { 
  background: green!important;
}

.....

Hi @sfhowes

Thanks for your input.

Further refinements are probably possible.

It works exactly as shown in your screenshot. Unfortunately, an "all day" event makes this solution less than ideal as an all-day event can itself occlude the color in that cell almost entirely.

I've got the developer tools (inspector) showing me (maybe not all?) selector names again, but any attempt to target the whole column seems to not work.

So . . . rather than targeting "calendar-header-container" (which seems to affect the "all day event" cell

I have tried targeting "multiday-column-bg-box" and "multiday-column-box-stack"

Unfortunately, neither have worked.

Clearly it should be possible as the default setup of Lightning is able to color the entire selected and/or current day column differently from others . . .

Any further insights would be appreciated!