I might be off base on this observation, but if I recall
correctly (from getting Oomph things to behave correctly with dark
theme), if one subclasses Composite, your subclass is no longer
styled as a composite.
But I think there is a misunderstanding (my fault). By "parent" in my answer, I meant the container control.
So, if drawing manually on a custom widget, instead of consuming widget.getBackground(), one can decide to consume widget.getParent().getBackground() and hope this one is properly configured.
But there could be better ways of doing this.