To fix this I did the following:
1. remove the compatibility directive:
2. add as an additional compiler option the following directive
3. change the namespace on my css file.
for the standard flex components:
and for my custom components
finally make your application's framework conform to Flex4SDK.
Thanks, possibly, although I think it is more related to the fact that each component on my .css file must explicitly state its appropriate namespace in order for the .css styles to be used...