The StyleManager has gone through some changes in Flex 4, primarily (it appears from my very cursory poking around) to support different modules having different CSS definitions.

So that’s a nice thing. There is one subtle and obscure change, though, involving class style declarations. Let’s say you want to programmatically change the colors of all your tooltips. The following, which worked under Flex 3, does not work under Flex 4. The variable css will be null every time.

var css:CSSStyleDeclaration = styleManager.getStyleDeclaration("Tooltip");

The  fix is easy: you need to include the namespace as well.

var css:CSSStyleDeclaration = styleManager.getStyleDeclaration("mx.controls.Tooltip");

That is all.