Good evening Annie,
You are correct, there seems to be some CSS missing from the last version.
Try adding the following CSS rule to your style.css file or in your Custom CSS metabox:
.tribe-mini-calendar-event .list-date {
border-right: 3px solid #666;
}
Another approach would be to use the following CSS rule instead
.tribe-mini-calendar-event .list-date .list-dayname {
box-sizing: border-box;
-moz-box-sizing: border-box;
-webkit-box-sizing: border-box;
}
Let me know how that goes.
Best regards,
Geoff B.