Fix broken demo, improve passthrough CSS (#879)

Fixes several issues for components using the `display: contents` technique:
- Introduce `--display` CSS property to make it possible to override internal display value of base part.
- Ensure `display: contents` is always applied with `!important` to avoid rendering glitches like the one here
- Ensure non-inherited custom properties are also inherited
- Ensure the `hidden` attribute still works.

docs/docs/tokens/focus.md

@@ -27,6 +27,7 @@ See your theme's focus ring in action by navigating this form example with your
 <style>
   form > * + * {
     display: block;
+    --display: block;
     width: fit-content;
     margin-block-start: var(--wa-space-m);
   }