Howdy Philippe,
Good question. You can change any CSS by adding it to your theme’s style.css file or using a plugin like Simple Custom CSS.
In this case, targeting a past event would look something like this:
.tribe-events-calendar td.tribe-events-past {
background: #333 !important; /* or whatever color you'd like */
}'
...and for the title background color:
.single-tribe_events .sidebar .tribe-events-single-section-title {
background-color: #333 !important; /* or whatever color you’d like */
}`
Will this work for you? Please let me know. 🙂
Cheers,
Geoff