I've noticed that formstyle UNIFIED is used sometimes. However I cannot find it part of the method description.
public static final StageStyle UNIFIED
Defines a Stage style with platform decorations and eliminates the border between client area and decorations. The client area background is unified with the decorations. This is a conditional feature. If the feature is not supported by the platform, this style downgrades to StageStyle.DECORATED
` NOTE: To see the effect the Scene covering the Stage should have Color.TRANSPARENT