Run SceneBuilder and load IssueTrackingLite.fxml.
Warnings ensue.

Note that, while annoying, these warnings do not cause any issues. The styles resolve themselves on the next CSS pass. I mention this because it could be lived with for 8u20.

I tracked the issue down to a call to applyCss() in WorkspaceControler#setPreviewStyleSheets. If I comment out that call, the warnings go away. _This is not a proposed fix_. This just confirms that it is the call to applyCss() that causes the warnings.

At the time the applyCss() method is called, the root of the workspace (a VBox node with style-class ".root.theme-presets") has cssFlag == REAPPLY. My guess is that the reapply was defered, and then applyCss was called on a child. applyCss has logic that looks for the top-most node that needs UPDATE. This should probably look for nodes that need either UPDATE or REAPPLY.