Hi Pat,
You can accomplish this with some fairly basic CSS. First, find and make a copy of /wp-content/plugins/the-events-calendar/resources/events.css and place the copy in an ‘events’ folder in your theme. This will allow you to make changes to the plugins CSS and not lose them the next time you update. Now, open the file in your favorite editor and add the following all the way at the bottom of the file (you may even want to comment this block to indicate they are your custom changes):
/* My Custom Events Plugin CSS */
.tribe-events-calendar-widget .daynum.has-events {
background: #ccc;
}
This will give any dates in the calendar widget that have events a light grey background. Of course this is a super basic example, there’s a lot more you could do and the skies the limit. I would suggest playing around with this a little and maybe reading up a little on CSS to get an idea of how to work with it.
I hope that helps!
Cheers,
Jonah