diff --git a/src/wp-includes/class-wp-theme-json-resolver.php b/src/wp-includes/class-wp-theme-json-resolver.php
index cf14486f6770d..c166283af30f1 100644
--- a/src/wp-includes/class-wp-theme-json-resolver.php
+++ b/src/wp-includes/class-wp-theme-json-resolver.php
@@ -246,8 +246,13 @@ public static function get_theme_data( $deprecated = array(), $options = array()
 		$options = wp_parse_args( $options, array( 'with_supports' => true ) );
 
 		if ( null === static::$theme || ! static::has_same_registered_blocks( 'theme' ) ) {
-			$theme_json_data = static::read_json_file( static::get_file_path_from_theme( 'theme.json' ) );
-			$theme_json_data = static::translate( $theme_json_data, wp_get_theme()->get( 'TextDomain' ) );
+			$theme_json_file = static::get_file_path_from_theme( 'theme.json' );
+			if ( '' !== $theme_json_file ) {
+				$theme_json_data = static::read_json_file( $theme_json_file );
+				$theme_json_data = static::translate( $theme_json_data, wp_get_theme()->get( 'TextDomain' ) );
+			} else {
+				$theme_json_data = array();
+			}
 
 			/**
 			 * Filters the data provided by the theme for global styles and settings.
@@ -259,20 +264,23 @@ public static function get_theme_data( $deprecated = array(), $options = array()
 			$theme_json      = apply_filters( 'wp_theme_json_data_theme', new WP_Theme_JSON_Data( $theme_json_data, 'theme' ) );
 			$theme_json_data = $theme_json->get_data();
 			static::$theme   = new WP_Theme_JSON( $theme_json_data );
-		}
-
-		if ( wp_get_theme()->parent() ) {
-			// Get parent theme.json.
-			$parent_theme_json_data = static::read_json_file( static::get_file_path_from_theme( 'theme.json', true ) );
-			$parent_theme_json_data = static::translate( $parent_theme_json_data, wp_get_theme()->parent()->get( 'TextDomain' ) );
-			$parent_theme           = new WP_Theme_JSON( $parent_theme_json_data );
 
-			/*
-			 * Merge the child theme.json into the parent theme.json.
-			 * The child theme takes precedence over the parent.
-			 */
-			$parent_theme->merge( static::$theme );
-			static::$theme = $parent_theme;
+			if ( wp_get_theme()->parent() ) {
+				// Get parent theme.json.
+				$parent_theme_json_file = static::get_file_path_from_theme( 'theme.json', true );
+				if ( '' !== $parent_theme_json_file ) {
+					$parent_theme_json_data = static::read_json_file( $parent_theme_json_file );
+					$parent_theme_json_data = static::translate( $parent_theme_json_data, wp_get_theme()->parent()->get( 'TextDomain' ) );
+					$parent_theme           = new WP_Theme_JSON( $parent_theme_json_data );
+
+					/*
+					 * Merge the child theme.json into the parent theme.json.
+					 * The child theme takes precedence over the parent.
+					 */
+					$parent_theme->merge( static::$theme );
+					static::$theme = $parent_theme;
+				}
+			}
 		}
 
 		if ( ! $options['with_supports'] ) {
@@ -403,6 +411,18 @@ public static function get_user_data_from_wp_global_styles( $theme, $create_post
 		if ( ! $theme instanceof WP_Theme ) {
 			$theme = wp_get_theme();
 		}
+
+		/*
+		 * Bail early if the theme does not support a theme.json.
+		 *
+		 * Since WP_Theme_JSON_Resolver::theme_has_support() only supports the active
+		 * theme, the extra condition for whether $theme is the active theme is
+		 * present here.
+		 */
+		if ( $theme->get_stylesheet() === get_stylesheet() && ! static::theme_has_support() ) {
+			return array();
+		}
+
 		$user_cpt         = array();
 		$post_type_filter = 'wp_global_styles';
 		$stylesheet       = $theme->get_stylesheet();
@@ -582,8 +602,8 @@ public static function get_user_global_styles_post_id() {
 	public static function theme_has_support() {
 		if ( ! isset( static::$theme_has_support ) ) {
 			static::$theme_has_support = (
-				is_readable( static::get_file_path_from_theme( 'theme.json' ) ) ||
-				is_readable( static::get_file_path_from_theme( 'theme.json', true ) )
+				static::get_file_path_from_theme( 'theme.json' ) !== '' ||
+				static::get_file_path_from_theme( 'theme.json', true ) !== ''
 			);
 		}
 
diff --git a/src/wp-includes/class-wp-theme-json.php b/src/wp-includes/class-wp-theme-json.php
index d0fcdeb52a3c6..df5bd20d84917 100644
--- a/src/wp-includes/class-wp-theme-json.php
+++ b/src/wp-includes/class-wp-theme-json.php
@@ -2939,7 +2939,8 @@ public function get_data() {
 	public function set_spacing_sizes() {
 		$spacing_scale = _wp_array_get( $this->theme_json, array( 'settings', 'spacing', 'spacingScale' ), array() );
 
-		if ( ! is_numeric( $spacing_scale['steps'] )
+		if ( ! isset( $spacing_scale['steps'] )
+			|| ! is_numeric( $spacing_scale['steps'] )
 			|| ! isset( $spacing_scale['mediumStep'] )
 			|| ! isset( $spacing_scale['unit'] )
 			|| ! isset( $spacing_scale['operator'] )
diff --git a/tests/phpunit/tests/theme/wpThemeJsonResolver.php b/tests/phpunit/tests/theme/wpThemeJsonResolver.php
index 524fade2d8f85..1cbc519255cf4 100644
--- a/tests/phpunit/tests/theme/wpThemeJsonResolver.php
+++ b/tests/phpunit/tests/theme/wpThemeJsonResolver.php
@@ -33,13 +33,6 @@ class Tests_Theme_wpThemeJsonResolver extends WP_UnitTestCase {
 	 */
 	private $orig_theme_dir;
 
-	/**
-	 * Queries.
-	 *
-	 * @var array
-	 */
-	private $queries = array();
-
 	/**
 	 * WP_Theme_JSON_Resolver::$blocks_cache property.
 	 *
@@ -105,7 +98,7 @@ public function set_up() {
 		add_filter( 'theme_root', array( $this, 'filter_set_theme_root' ) );
 		add_filter( 'stylesheet_root', array( $this, 'filter_set_theme_root' ) );
 		add_filter( 'template_root', array( $this, 'filter_set_theme_root' ) );
-		$this->queries = array();
+
 		// Clear caches.
 		wp_clean_themes_cache();
 		unset( $GLOBALS['wp_themes'] );
@@ -129,13 +122,6 @@ public function filter_set_locale_to_polish() {
 		return 'pl_PL';
 	}
 
-	function filter_db_query( $query ) {
-		if ( preg_match( '#post_type = \'wp_global_styles\'#', $query ) ) {
-			$this->queries[] = $query;
-		}
-		return $query;
-	}
-
 	/**
 	 * @ticket 52991
 	 * @ticket 54336
@@ -634,17 +620,26 @@ function test_merges_child_theme_json_into_parent_theme_json() {
 	 * @covers WP_Theme_JSON_Resolver::get_user_data_from_wp_global_styles
 	 */
 	function test_get_user_data_from_wp_global_styles_does_not_use_uncached_queries() {
+		// Switch to a theme that does have support.
+		switch_theme( 'block-theme' );
 		wp_set_current_user( self::$administrator_id );
 		$theme = wp_get_theme();
 		WP_Theme_JSON_Resolver::get_user_data_from_wp_global_styles( $theme );
-		add_filter( 'query', array( $this, 'filter_db_query' ) );
-		$query_count = count( $this->queries );
+		$global_styles_query_count = 0;
+		add_filter(
+			'query',
+			function( $query ) use ( &$global_styles_query_count ) {
+				if ( preg_match( '#post_type = \'wp_global_styles\'#', $query ) ) {
+					$global_styles_query_count++;
+				}
+				return $query;
+			}
+		);
 		for ( $i = 0; $i < 3; $i++ ) {
 			WP_Theme_JSON_Resolver::get_user_data_from_wp_global_styles( $theme );
 			WP_Theme_JSON_Resolver::clean_cached_data();
 		}
-		$query_count = count( $this->queries ) - $query_count;
-		$this->assertSame( 0, $query_count, 'Unexpected SQL queries detected for the wp_global_style post type prior to creation.' );
+		$this->assertSame( 0, $global_styles_query_count, 'Unexpected SQL queries detected for the wp_global_style post type prior to creation.' );
 
 		$user_cpt = WP_Theme_JSON_Resolver::get_user_data_from_wp_global_styles( $theme );
 		$this->assertEmpty( $user_cpt, 'User CPT is expected to be empty.' );
@@ -652,40 +647,64 @@ function test_get_user_data_from_wp_global_styles_does_not_use_uncached_queries(
 		$user_cpt = WP_Theme_JSON_Resolver::get_user_data_from_wp_global_styles( $theme, true );
 		$this->assertNotEmpty( $user_cpt, 'User CPT is expected not to be empty.' );
 
-		$query_count = count( $this->queries );
+		$global_styles_query_count = 0;
 		for ( $i = 0; $i < 3; $i ++ ) {
 			$new_user_cpt = WP_Theme_JSON_Resolver::get_user_data_from_wp_global_styles( $theme );
 			WP_Theme_JSON_Resolver::clean_cached_data();
 			$this->assertSameSets( $user_cpt, $new_user_cpt, "User CPTs do not match on run {$i}." );
 		}
-		$query_count = count( $this->queries ) - $query_count;
-		$this->assertSame( 1, $query_count, 'Unexpected SQL queries detected for the wp_global_style post type after creation.' );
+		$this->assertSame( 1, $global_styles_query_count, 'Unexpected SQL queries detected for the wp_global_style post type after creation.' );
 	}
 
 	/**
 	 * @covers WP_Theme_JSON_Resolver::get_user_data_from_wp_global_styles
 	 */
 	function test_get_user_data_from_wp_global_styles_does_not_use_uncached_queries_for_logged_out_users() {
+		// Switch to a theme that does have support.
+		switch_theme( 'block-theme' );
 		$theme = wp_get_theme();
 		WP_Theme_JSON_Resolver::get_user_data_from_wp_global_styles( $theme );
-		add_filter( 'query', array( $this, 'filter_db_query' ) );
-		$query_count = count( $this->queries );
+		$query_count = get_num_queries();
 		for ( $i = 0; $i < 3; $i++ ) {
 			WP_Theme_JSON_Resolver::get_user_data_from_wp_global_styles( $theme );
 			WP_Theme_JSON_Resolver::clean_cached_data();
 		}
-		$query_count = count( $this->queries ) - $query_count;
+		$query_count = get_num_queries() - $query_count;
 		$this->assertSame( 0, $query_count, 'Unexpected SQL queries detected for the wp_global_style post type prior to creation.' );
 
 		$user_cpt = WP_Theme_JSON_Resolver::get_user_data_from_wp_global_styles( $theme );
 		$this->assertEmpty( $user_cpt, 'User CPT is expected to be empty.' );
 	}
 
+	/**
+	 * @ticket 56945
+	 * @covers WP_Theme_JSON_Resolver::get_user_data_from_wp_global_styles
+	 */
+	function test_get_user_data_from_wp_global_styles_does_not_run_for_theme_without_support() {
+		// The 'default' theme does not support theme.json.
+		switch_theme( 'default' );
+		wp_set_current_user( self::$administrator_id );
+		$theme = wp_get_theme();
+
+		$start_queries = get_num_queries();
+
+		// When theme.json is not supported, the method should not run a query and always return an empty result.
+		$user_cpt = WP_Theme_JSON_Resolver::get_user_data_from_wp_global_styles( $theme );
+		$this->assertEmpty( $user_cpt, 'User CPT is expected to be empty.' );
+		$this->assertSame( 0, get_num_queries() - $start_queries, 'Unexpected SQL query detected for theme without theme.json support.' );
+
+		$user_cpt = WP_Theme_JSON_Resolver::get_user_data_from_wp_global_styles( $theme, true );
+		$this->assertEmpty( $user_cpt, 'User CPT is expected to be empty.' );
+		$this->assertSame( 0, get_num_queries() - $start_queries, 'Unexpected SQL query detected for theme without theme.json support.' );
+	}
+
 	/**
 	 * @ticket 55392
 	 * @covers WP_Theme_JSON_Resolver::get_user_data_from_wp_global_styles
 	 */
 	function test_get_user_data_from_wp_global_styles_does_exist() {
+		// Switch to a theme that does have support.
+		switch_theme( 'block-theme' );
 		$theme = wp_get_theme();
 		$post1 = WP_Theme_JSON_Resolver::get_user_data_from_wp_global_styles( $theme, true );
 		$this->assertIsArray( $post1 );
@@ -701,6 +720,8 @@ function test_get_user_data_from_wp_global_styles_does_exist() {
 	 * @covers WP_Theme_JSON_Resolver::get_user_data_from_wp_global_styles
 	 */
 	function test_get_user_data_from_wp_global_styles_create_post() {
+		// Switch to a theme that does have support.
+		switch_theme( 'block-theme' );
 		$theme = wp_get_theme( 'testing' );
 		$post1 = WP_Theme_JSON_Resolver::get_user_data_from_wp_global_styles( $theme );
 		$this->assertIsArray( $post1 );
@@ -718,6 +739,8 @@ function test_get_user_data_from_wp_global_styles_create_post() {
 	 * @covers WP_Theme_JSON_Resolver::get_user_data_from_wp_global_styles
 	 */
 	function test_get_user_data_from_wp_global_styles_filter_state() {
+		// Switch to a theme that does have support.
+		switch_theme( 'block-theme' );
 		$theme = wp_get_theme( 'foo' );
 		$post1 = WP_Theme_JSON_Resolver::get_user_data_from_wp_global_styles( $theme, true, array( 'publish' ) );
 		$this->assertIsArray( $post1 );
@@ -749,4 +772,29 @@ function test_get_theme_data_theme_supports_overrides_theme_json() {
 		$line_height      = $current_settings['typography']['lineHeight'];
 		$this->assertTrue( $line_height, 'lineHeight setting after add_theme_support() should be true.' );
 	}
+
+	/**
+	 * @ticket 56945
+	 * @covers WP_Theme_JSON_Resolver::get_theme_data
+	 */
+	function test_get_theme_data_does_not_parse_theme_json_if_not_present() {
+		// The 'default' theme does not support theme.json.
+		switch_theme( 'default' );
+
+		$theme_json_resolver = new WP_Theme_JSON_Resolver();
+
+		// Force-unset $i18n_schema property to "unload" translation schema.
+		$property = new ReflectionProperty( $theme_json_resolver, 'i18n_schema' );
+		$property->setAccessible( true );
+		$property->setValue( null );
+
+		// A completely empty theme.json data set still has the 'version' key when parsed.
+		$empty_theme_json = array( 'version' => WP_Theme_JSON::LATEST_SCHEMA );
+
+		// Call using 'with_supports' set to false, so that the method only considers theme.json.
+		$theme_data = $theme_json_resolver->get_theme_data( array(), array( 'with_supports' => false ) );
+		$this->assertInstanceOf( 'WP_Theme_JSON', $theme_data, 'Theme data should be an instance of WP_Theme_JSON.' );
+		$this->assertSame( $empty_theme_json, $theme_data->get_raw_data(), 'Theme data should be empty without theme support.' );
+		$this->assertNull( $property->getValue(), 'Theme i18n schema should not have been loaded without theme support.' );
+	}
 }