I’m a bit late on this, I know, but the issue is because there is no class assigned to upcoming events on the calendar, in an apparent lack of foresight on the part of the plugin developer. Rather than assign upcoming events an class based on the database information, they are given inline styling of “cursor: pointer” and given an event bind for the Thickbox JQuery plugin. This, of course, makes it difficult to style. There are some options, however.
1. You can edit the actual plugin files to assign a class to the table cell or span in the table corresponding to events in your events database. This is the most complicated method and prevents you from being able to update the plugin without losing your changes, but is the ideal solution for standards concern.
2. You can use #wp-calendar td span[style="cursor: pointer;"]
as a selector in CSS to assign styles to the events. Make sure to use display: block; width: 100%; height: 100%
with this, if you want to use any background colors, since the span needs to fill its table cell. Also, this method doesn’t work with some older browsers that don’t support attribute selectors in CSS.
3. You can use JavaScript (or JQuery, since it’s already being used for the plugin) to target any cell within the table containing a span child with “cursor: pointer” css, and style it that way. Something like this:
$('#wp-calendar td span[style="cursor: pointer;"]').addClass('upcoming-event');
Then, you’ll just style the upcoming-event
class whatever way you want.