If you're looking for a solution to the Flex 4.0 compiler warning "The style 'dropShadowVisible' is only supported by type 'mx.controls.List' with the theme(s) 'spark'.", here it is (see the comment from Carlos Bonilla): http://bugs.adobe.com/jira/browse/SDK-25720
The fix is a one-liner, and just requires going into the SDK and commenting out a line in a CSS file.
I ran into this issue months ago with a Flex 4.0 project and applied a workaround, which basically consisted of ignoring all CSS-related warnings in the Flash Builder compiler. This didn't feel quite right, but I didn't have time to keep screwing with it.
Fast-forward to last week, and a separate project, and here it was again. I like to have a clean "Problems" view when I'm working in a project, so I found this menacing. I needed a proper fix once and for all.
The solution is posted in the Adobe Forums for the world to see, but it took me a while to find it. The only reason for this post is the hope I can spare some poor soul out there the frustration of tracking it down.