Disable layout styles in classic themes

Author: youknowriad

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

@@ -1642,6 +1642,12 @@ protected function get_layout_styles( $block_metadata, $types = array() ) {
 			return $block_rules;
 		}
 
+		// Skip outputting layout styles for classic themes without theme.json.
+		// These styles are only relevant for block themes and themes using theme.json.
+		if ( ! wp_is_block_theme() && ! wp_theme_has_theme_json() ) {
+			return $block_rules;
+		}
+
 		if ( isset( $block_metadata['name'] ) ) {
 			$block_type = WP_Block_Type_Registry::get_instance()->get_registered( $block_metadata['name'] );
 			if ( ! block_has_support( $block_type, 'layout', false ) && ! block_has_support( $block_type, '__experimentalLayout', false ) ) {
@@ -3107,9 +3113,13 @@ public function get_root_layout_rules( $selector, $block_metadata ) {
 			$css .= '.has-global-padding :where(:not(.alignfull.is-layout-flow) > .has-global-padding:not(.wp-block-block, .alignfull)) > .alignfull { margin-left: 0; margin-right: 0; }';
 		}
 
-		$css .= '.wp-site-blocks > .alignleft { float: left; margin-right: 2em; }';
-		$css .= '.wp-site-blocks > .alignright { float: right; margin-left: 2em; }';
-		$css .= '.wp-site-blocks > .aligncenter { justify-content: center; margin-left: auto; margin-right: auto; }';
+		// Skip outputting alignment styles for classic themes without theme.json.
+		// These styles target .wp-site-blocks which is only used by block themes.
+		if ( wp_is_block_theme() || wp_theme_has_theme_json() ) {
+			$css .= '.wp-site-blocks > .alignleft { float: left; margin-right: 2em; }';
+			$css .= '.wp-site-blocks > .alignright { float: right; margin-left: 2em; }';
+			$css .= '.wp-site-blocks > .aligncenter { justify-content: center; margin-left: auto; margin-right: auto; }';
+		}
 
 		// Block gap styles will be output unless explicitly set to `null`. See static::PROTECTED_PROPERTIES.
 		if ( isset( $this->theme_json['settings']['spacing']['blockGap'] ) ) {