Help to hide "All-day" badge

I saw a topic where this was addressed with the following piece of code added to the css editor, but it doesn’t seem to work in my theme (a child of twenty-eleven).

.ai1ec-allday-badge {
display: none;
}

Can someone help me with the correct way to write this code for my theme? Some events I’m posting do not have stated start and end times, but neither are they “all day”.

https://www.ads-software.com/plugins/all-in-one-event-calendar/