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What are the downsides to using Spark over Halo in Flex?
What is the difference between Halo and Spark in Flex3 and Flex4?
Spark is a new theme introduced (in addition to Halo) with Flash builder 4 (Flex4). These two themes support different kinds of styles. With Flex4, you can choose which one you want to use depending on design considerations for your application. Spark is more convenient if you want to use custom skins on your components; Halo, on the other hand, has more built-in styles for components.