The calendar can be a great help when deciding on a theme for your event. If you're fresh out of ideas, see what holidays fall around the date of your event. You may even be able to reschedule your venue to better fit with a holiday. Some great calendar-based gala ideas include winter wonderland, Chinese New Year, Valentine's Day, Cinco de Mayo, The Kentucky Derby or even Earth Day.
If you still haven't found anything your committee can agree on, try taking more of a risk with your theme. Maybe your guests would go for a masquerade event, where the focus is more on the guests' attire than the overall décor. This theme would allow you to keep the decorations simpler, so you can focus your money on the food, drinks or entertainment. Or take inspiration from current trends and throw a "Top Chef" cook-off or "Dancing with the Stars" event. Your entertainment would include local celebrities or possibly even some of the less fortunate who your organization serves.
Don't forget, your cause is the best theme of all. If you're set on an idea that does not showcase your cause, try to find a way to weave it into your theme of choice. Whether it's having the children you serve write a valentine to each guest at your Valentine's Day-themed event or planning the entire night around personal stories from those you serve, if your guests are touched, they'll be more inclined to open their wallets.