Add negative margins to top-level group and cover blocks (#336)

* Try making full-width backgrounds easier.

* Tidy up default left-right padding for group blocks with a background color.

* Make this work with template parts.

style.css

@@ -99,6 +99,7 @@ a:active {
 body > .is-root-container,
 .edit-post-visual-editor__post-title-wrapper,
 .wp-block-group.alignfull,
+.wp-block-group.has-background,
 .wp-block-cover.alignfull,
 .is-root-container .wp-block[data-align="full"] > .wp-block-group,
 .is-root-container .wp-block[data-align="full"] > .wp-block-cover {
@@ -107,6 +108,14 @@ body > .is-root-container,
 }
 
 .wp-site-blocks .alignfull,
+.wp-site-blocks > .wp-block-group.has-background,
+.wp-site-blocks > .wp-block-cover,
+.wp-site-blocks > .wp-block-template-part > .wp-block-group.has-background,
+.wp-site-blocks > .wp-block-template-part > .wp-block-cover,
+body > .is-root-container > .wp-block-group.has-background,
+body > .is-root-container > .wp-block-cover,
+body > .is-root-container > .wp-block-template-part > .wp-block-group.has-background,
+body > .is-root-container > .wp-block-template-part > .wp-block-cover,
 .is-root-container .wp-block[data-align="full"] {
 	margin-left: calc(-1 * var(--wp--custom--spacing--outer)) !important;
 	margin-right: calc(-1 * var(--wp--custom--spacing--outer)) !important;