All themes must have a customized Share Card. This ensures an Share Card functions properly when your page is shared on social networks. 

Solution:

To properly create and style your event card, you’ll need to go into the Settings tab of your event. From there, head to the Sharing Options and click the Edit Share Card button. This will bring you to the Event Card Workspace, where you can stylize your Share Card to make it consistent with your page's aesthetic and theme.

*Note that you have to save your entire page at the top for the Share Card to populate correctly.