Hi, Petra.
You posted in our Pre-Sales forum from a user account that doesn’t have any purchases associated with it.
We don’t answer support questions for The Events Calendar core/free or Event Tickets core/free or their paid add-ons in the Pre-Sales forum.
If you made your purchase(s) through a different user account, please login to that account and then post in one of the paid forums.
Thank you for your understanding and thank you for using our plugins.
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Possibly helpful references:
We ensure compatibility with WordPress default themes like Twenty Sixteen, Twenty Seventeen, etc. Other than that, we typically state that our stuff works with any theme that follows WP’s best practices. There are also other themes that either state they’re compatible (their claim, not ours) and some even have specific integrations and custom views for our plugin’s pages. You can find some of these at ThemeForest.net.
I hope this helps!