Follow-ups from review of #940. A document handed to the editor with a theme keeps its first page's stored background; only a page without one takes the theme default. Canvas layers are numbered from the page they go on rather than the page on screen. Script strokes are copied with copyStrokes so the node owns its nested paint values. CanvasHelper switches only a fresh document (one empty page on the exact dark default) to the light page, and the round-trip test imports SceneGraph from its own package.
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