backport WordPress/gutenberg#46571

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

@@ -1006,6 +1006,27 @@ public function get_stylesheet( $types = array( 'variables', 'styles', 'presets'
 		return $stylesheet;
 	}
 
+	/**
+	 * Processes the CSS, to apply nesting.
+	 *
+	 * @param string $css      The CSS to process.
+	 * @param string $selector The selector to nest.
+	 *
+	 * @return string The processed CSS.
+	 */
+	public 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.
 	 *

tests/phpunit/tests/theme/wpThemeJson.php

@@ -4624,4 +4624,55 @@ public function data_custom_css_for_user_caps() {
 			),
 		);
 	}
+
+	/**
+	 * @dataProvider data_process_blocks_custom_css
+	 *
+	 * @param array  $input    An array containing the selector and css to test.
+	 * @param string $expected Expected results.
+	 */
+	public function test_process_blocks_custom_css( $input, $expected ) {
+		$theme_json = new WP_Theme_JSON_Gutenberg(
+			array(
+				'version' => WP_Theme_JSON_Gutenberg::LATEST_SCHEMA,
+				'styles'  => array(),
+			)
+		);
+
+		$this->assertEquals( $expected, $theme_json->process_blocks_custom_css( $input['css'], $input['selector'] ) );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array[]
+	 */
+	public function data_process_blocks_custom_css() {
+		return array(
+			// Simple CSS without any child selectors.
+			'no child selectors'                => array(
+				'input'    => array(
+					'selector' => '.foo',
+					'css'      => 'color: red; margin: auto;',
+				),
+				'expected' => '.foo{color: red; margin: auto;}',
+			),
+			// CSS with child selectors.
+			'with children'                     => array(
+				'input'    => array(
+					'selector' => '.foo',
+					'css'      => 'color: red; margin: auto; & .bar{color: blue;}',
+				),
+				'expected' => '.foo{color: red; margin: auto;}.foo .bar{color: blue;}',
+			),
+			// CSS with child selectors and pseudo elements.
+			'with children and pseudo elements' => array(
+				'input'    => array(
+					'selector' => '.foo',
+					'css'      => 'color: red; margin: auto; & .bar{color: blue;} &::before{color: green;}',
+				),
+				'expected' => '.foo{color: red; margin: auto;}.foo .bar{color: blue;}.foo::before{color: green;}',
+			),
+		);
+	}
 }