There are many things to consider before answering this question.
What is the event for?
Is it a fundraiser? A fundraiser for a particular person or cause may have an obvious inclination to a theme. For a person, focus on telling their story. What makes them special? How will having the funds raised change their life? For a cause, where is the money going to? How many people will this help? The cause itself is a theme, so your focus may be on ways to get the most donations and interest for the cause. If you are raising money for a particular organization, who does that organization serve? What are their values? What is their mission? All of these things give you insight into how to frame your event.
Are you honoring someone? Why? Whom have they affected? Is that person still alive? Is it a surprise? Will there be speakers to honor that person? Are they receiving an award? Do you know their likes and what is meaningful to them?
Are you celebrating something? I love celebrations because theme or not, they are filled with joy, laughter, and fun. The word celebrate has such a positive connotation. For example a person may ask me to help plan a funeral and another ask me to celebrate the life of a family member who has just passed away. They invoke very different responses and I react in turn. Celebration is a very vague term however, so I don’t get too excited until I know exactly what we are celebrating. Weddings, Anniversaries, Baptisms, Birthdays, Graduations and Cotillions (Coming of Age Parties) can all be lumped in this category. As previously mentioned, funerals occasionally can be too. All of these events however, focus on a unit (person or couple). Since it’s all about that person(s), it’s very easy to find what is meaningful for them and frame your focus around that.