Fix checkboxes for postmeta. (#18108)

packages/edit-post/src/components/meta-boxes/meta-boxes-area/style.scss

@@ -81,6 +81,25 @@
 	.is-hidden {
 		display: none;
 	}
+
+
+	// Until checkboxes WordPress-wide are updated to match the new style,
+	// checkboxes used in metaboxes have to be slightly unstyled here.
+	// @todo: remove this entire rule once checkboxes are the same everywhere.
+	// See: https://github.com/WordPress/gutenberg/issues/18053
+	.postbox-container .postbox input[type="checkbox"],
+	.postbox-container .postbox input[type="radio"] {
+		border: $border-width solid $dark-gray-300;
+
+		&:checked {
+			background: $white;
+			border-color: $dark-gray-300;
+		}
+
+		&::before {
+			margin: -3px -4px;
+		}
+	}
 }
 
 .edit-post-meta-boxes-area__clear {