Hey Sylvie!
Thanks for reaching out.
Our plugins are all compatible with any standards-compliant WordPress theme. If a theme breaks WordPress coding standards, or otherwise has bugs in it, then it could break plugins like The Events Calendar. 🙁
I do not know how the “Ultimatum” theme fares on these fronts, but I would like to mention that The Events Calendar is a free plugin. So if the Ultimatum theme has a good refund policy, you may find it best to install The Events Calendar for free; install the Ultimatum theme; and see how they behave together.
If problems arise, then you can know this without having to buy any of our premium add-ons.
If you do buy any of our premium add-ons and notice conflicts, please note that we are happy to refund anything—for any reason—within thirty days of the date of purchase. You can read more about our refund policy here.
So, to recap: if the Ultimatum theme follows WordPress Coding Standards and does not have any bugs in it, then our plugins will work fine within it. If it has either problem or some other problem, then you can get a full refund from us for anything your happen to purchase.
I hope this information helps you proceed!
Sincerely,
George