The editor's CSS parser drops :root, html and body rules, so an
imported site lost its variables, page background and fonts and came
up white. Those rules are now split out and carried verbatim: injected
into the canvas (as `html body` so they outrank the editor's base
sheet), stored with the draft, and emitted ahead of the editor's CSS on
export. The source page's <html> attributes (lang, data-theme, class)
travel the same way, since themed sites key their variables on them.
Templates reset all of it.
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