Inject theme stylesheet value as template part theme attribute (#53423)

Co-authored-by: Sarah Norris <sarah@sekai.co.uk>

packages/block-library/src/pattern/edit.js

@@ -8,6 +8,7 @@ import {
 	store as blockEditorStore,
 	useBlockProps,
 } from '@wordpress/block-editor';
+import { store as coreStore } from '@wordpress/core-data';
 
 const PatternEdit = ( { attributes, clientId } ) => {
 	const selectedPattern = useSelect(
@@ -18,12 +19,42 @@ const PatternEdit = ( { attributes, clientId } ) => {
 		[ attributes.slug ]
 	);
 
+	const currentThemeStylesheet = useSelect(
+		( select ) => select( coreStore ).getCurrentTheme().stylesheet
+	);
+
 	const { replaceBlocks, __unstableMarkNextChangeAsNotPersistent } =
 		useDispatch( blockEditorStore );
 	const { setBlockEditingMode } = useDispatch( blockEditorStore );
 	const { getBlockRootClientId, getBlockEditingMode } =
 		useSelect( blockEditorStore );
 
+	function injectThemeAttributeInBlockTemplateContent( block ) {
+		if (
+			block.innerBlocks.find(
+				( innerBlock ) => innerBlock.name === 'core/template-part'
+			)
+		) {
+			block.innerBlocks = block.innerBlocks.map( ( innerBlock ) => {
+				if (
+					innerBlock.name === 'core/template-part' &&
+					innerBlock.attributes.theme === undefined
+				) {
+					innerBlock.attributes.theme = currentThemeStylesheet;
+				}
+				return innerBlock;
+			} );
+		}
+
+		if (
+			block.name === 'core/template-part' &&
+			block.attributes.theme === undefined
+		) {
+			block.attributes.theme = currentThemeStylesheet;
+		}
+		return block;
+	}
+
 	// Run this effect when the component loads.
 	// This adds the Pattern's contents to the post.
 	// This change won't be saved.
@@ -40,7 +71,9 @@ const PatternEdit = ( { attributes, clientId } ) => {
 				// Clone blocks from the pattern before insertion to ensure they receive
 				// distinct client ids. See https://github.com/WordPress/gutenberg/issues/50628.
 				const clonedBlocks = selectedPattern.blocks.map( ( block ) =>
-					cloneBlock( block )
+					cloneBlock(
+						injectThemeAttributeInBlockTemplateContent( block )
+					)
 				);
 				const rootEditingMode = getBlockEditingMode( rootClientId );
 				// Temporarily set the root block to default mode to allow replacing the pattern.

packages/block-library/src/pattern/index.php

@@ -41,7 +41,7 @@ function render_block_core_pattern( $attributes ) {
 	}
 
 	$pattern = $registry->get_registered( $slug );
-	$content = $pattern['content'];
+	$content = _inject_theme_attribute_in_block_template_content( $pattern['content'] );
 
 	$gutenberg_experiments = get_option( 'gutenberg-experiments' );
 	if ( $gutenberg_experiments && ! empty( $gutenberg_experiments['gutenberg-auto-inserting-blocks'] ) ) {