::backdrop pseudo-element support for inline SVG mode

[yhatt]

::backdrop pseudo-element can define styles for the background if in full-screen mode. This is helpful to change the color of letterboxes.

Marpit inline SVG mode is treating the parent <svg> element as the viewbox for scaling, so we can regard it as the backdrop of the slide element. Redirecting section::backdrop selector to the parent SVG will be useful for styling the letterbox.

/* @theme marpit-theme */

::backdrop {
  background: beige;
}

Related: marp-team/marp-cli#358, marp-team/marp#183

ToDo

• Keep section::backdrop selector for an original usage
• Consider how to handle classes for section in the parent SVG element
• Take <style scoped> into account
• Prevent chaining combinators from the ::backdrop selector to prevent circulate selector