Root style not applied without children blElement

[labordep]

To reproduce :

Theme :

ToMyDarkTheme>>defaultStampList
"..."
ToStyleStamp name: #'background-color' value: Color blue
"..."

StyleSheet :

rootPaneRules

	^ { ((self id: #'Space root') style: [ :sr |
		   sr
			   write: (self property: #background)
			   with: [ :e | e stampValue: #'background-color' ] ]) }

UI code :

| space |
	space := BlSpace new.
	space toTheme: ToMyDarkTheme new.
	
	space show.
	^ space

The background of the root not changed.
But when I add a BlElement into the root, the background is changed.

[labordep]

Concern SteelSheet branch of course.