Sidebar container block: Allow themes to override top margin

Reduce specificity of selector so that settings in the theme.json can override.

Fixes an issue with the top position of the ToC in Developer Resources.

mu-plugins/blocks/sidebar-container/postcss/style.pcss

@@ -102,6 +102,6 @@ main .wp-block-wporg-sidebar-container {
 }
 
 /* Set up the custom properties. These can be overridden by settings in theme.json. */
-:where(body) {
+:where(html) {
 	--wp--custom--wporg-sidebar-container--spacing--margin--top: var(--wp--preset--spacing--edge-space);
 }