Trying to style both modes of the text editor with the same style sheet
is proving to be more trouble than it’s worth. This prepares the shadow
DOM style sheet to diverge more radically to enable the background color
to be overridden from the outside more easily.
This prevents the need for a :host pseudo-class in the editor CSS which
breaks linting. It also fits selectors targeting the host element in a
more intuitive spot in the cascade.
