[whatwg/fullscreen] Impossible to customize the style of a dialog's ::backdrop residing inside a Shadow DOM. (#124)

**Note:** Copied from https://github.com/whatwg/html/issues/3601. Please continue discussion here.

More context at https://bugs.chromium.org/p/chromium/issues/detail?id=827397.

### Minimal repro 1
This just showcases that any CSS variable is ignored from ::backdrop elements, even without Shadow DOM usage.
https://jsfiddle.net/1zevfdce/3/

### Minimal repro 2
Showcases the problem when a` <dialog>` resides inside a Shadow DOM.
https://jsfiddle.net/o1trLoqL/2/

Note that the [spec](https://fullscreen.spec.whatwg.org/#::backdrop-pseudo-element) mentions the following

"It does not inherit from any element and is not inherited from. No restrictions are made on what properties apply to this pseudo-element either."

Is that the reason for the current behavior? If yes, should the spec be revised, such that customizing the style of a ::backdrop element is possible even if it resides inside a Shadow DOM? Or is there already a viable workaround?

cc @annevk @foolip @domenic @TakayoshiKochi

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