Clicking one option or the other will add a border or a highlight to the target element selected under Core Properties:
This is what a border looks like:
This is a highlight:
You can change the element that is highlighted (or has a border) by going to the Events section and clicking Set background color (or Set border).
To change the element, click Re-select element and select the element to be highlighted.
To delete a background color or border, click the trash icon next to the event: