Editor: Add missing WP_Theme_JSON::process_blocks_custom_css() method.

Follow up to [55192].

Props aristath, mamaduka, mukesh27, hellofromtonya.
Fixes #57621.


git-svn-id: https://develop.svn.wordpress.org/trunk@55216 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-theme-json.php

@@ -1006,12 +1006,34 @@ class WP_Theme_JSON {
 		return $stylesheet;
 	}
 
+	/**
+	 * Processes the CSS, to apply nesting.
+	 *
+	 * @since 6.2.0
+	 *
+	 * @param string $css      The CSS to process.
+	 * @param string $selector The selector to nest.
+	 * @return string The processed CSS.
+	 */
+	protected function process_blocks_custom_css( $css, $selector ) {
+		$processed_css = '';
+
+		// Split CSS nested rules.
+		$parts = explode( '&', $css );
+		foreach ( $parts as $part ) {
+			$processed_css .= ( ! str_contains( $part, '{' ) )
+				? trim( $selector ) . '{' . trim( $part ) . '}' // If the part doesn't contain braces, it applies to the root level.
+				: trim( $selector . $part ); // Prepend the selector, which effectively replaces the "&" character.
+		}
+		return $processed_css;
+	}
+
 	/**
 	 * Returns the global styles custom css.
 	 *
 	 * @since 6.2.0
 	 *
-	 * @return string
+	 * @return string The global styles custom CSS.
 	 */
 	public function get_custom_css() {
 		// Add the global styles root CSS.