If you develop for a system which permits theming, you have to include that use case in your design. You can’t complain about users making use of system features when you develop for that system.
Who are you to tell the app developer what they're require to support? It's open source. They don't owe you anything. They don't have to make their software work on Thursdays or work in Canada, let alone survive contact with a hundred weird and unpredictable themes.
