In Flex 3, the styles defined using the [Style] metadata were validated by the flex compiler, but styles defined in css files were not validated..
Flex 4 has multiple themes:
Spark: The default theme for Flex 4. It contains the skins for Spark and MX Components
Halo: The default theme for MX components (if you set -compatibility-version=3.0.0)
Some style . . . → Read More: Styles, Themes, and the stricter Compiler
