diff --git a/wp-includes/class-wp-duotone.php b/wp-includes/class-wp-duotone.php
index 40e9f7548b..e8b7735457 100644
--- a/wp-includes/class-wp-duotone.php
+++ b/wp-includes/class-wp-duotone.php
@@ -809,12 +809,13 @@ class WP_Duotone {
 	 * @internal
 	 *
 	 * @since 6.3.0
+	 * @since 6.6.0 Replaced body selector with `WP_Theme_JSON::ROOT_CSS_PROPERTIES_SELECTOR`.
 	 *
 	 * @param array $sources The duotone presets.
 	 * @return string The CSS for global styles.
 	 */
 	private static function get_global_styles_presets( $sources ) {
-		$css = 'body{';
+		$css = WP_Theme_JSON::ROOT_CSS_PROPERTIES_SELECTOR . '{';
 		foreach ( $sources as $filter_id => $filter_data ) {
 			$slug              = $filter_data['slug'];
 			$colors            = $filter_data['colors'];
diff --git a/wp-includes/class-wp-theme-json.php b/wp-includes/class-wp-theme-json.php
index 39e3581c18..e039fdbb9a 100644
--- a/wp-includes/class-wp-theme-json.php
+++ b/wp-includes/class-wp-theme-json.php
@@ -38,6 +38,14 @@ class WP_Theme_JSON {
 	 */
 	protected static $blocks_metadata = array();
 
+	/**
+	 * The CSS selector for the top-level preset settings.
+	 *
+	 * @since 6.6.0
+	 * @var string
+	 */
+	const ROOT_CSS_PROPERTIES_SELECTOR = ':root';
+
 	/**
 	 * The CSS selector for the top-level styles.
 	 *
@@ -1708,6 +1716,7 @@ class WP_Theme_JSON {
 	 *
 	 * @since 5.8.0
 	 * @since 5.9.0 Added the `$origins` parameter.
+	 * @since 6.6.0 Added check for root CSS properties selector.
 	 *
 	 * @param array    $settings Settings to process.
 	 * @param string   $selector Selector wrapping the classes.
@@ -1715,7 +1724,7 @@ class WP_Theme_JSON {
 	 * @return string The result of processing the presets.
 	 */
 	protected static function compute_preset_classes( $settings, $selector, $origins ) {
-		if ( static::ROOT_BLOCK_SELECTOR === $selector ) {
+		if ( static::ROOT_BLOCK_SELECTOR === $selector || static::ROOT_CSS_PROPERTIES_SELECTOR === $selector ) {
 			/*
 			 * Classes at the global level do not need any CSS prefixed,
 			 * and we don't want to increase its specificity.
@@ -2233,7 +2242,7 @@ class WP_Theme_JSON {
 		// Top-level.
 		$nodes[] = array(
 			'path'     => array( 'settings' ),
-			'selector' => static::ROOT_BLOCK_SELECTOR,
+			'selector' => static::ROOT_CSS_PROPERTIES_SELECTOR,
 		);
 
 		// Calculate paths for blocks.
@@ -2614,6 +2623,7 @@ class WP_Theme_JSON {
 	 * Outputs the CSS for layout rules on the root.
 	 *
 	 * @since 6.1.0
+	 * @since 6.6.0 Use `ROOT_CSS_PROPERTIES_SELECTOR` for CSS custom properties.
 	 *
 	 * @param string $selector The root node selector.
 	 * @param array  $block_metadata The metadata for the root block.
@@ -2624,16 +2634,6 @@ class WP_Theme_JSON {
 		$settings         = isset( $this->theme_json['settings'] ) ? $this->theme_json['settings'] : array();
 		$use_root_padding = isset( $this->theme_json['settings']['useRootPaddingAwareAlignments'] ) && true === $this->theme_json['settings']['useRootPaddingAwareAlignments'];
 
-		/*
-		* Reset default browser margin on the root body element.
-		* This is set on the root selector **before** generating the ruleset
-		* from the `theme.json`. This is to ensure that if the `theme.json` declares
-		* `margin` in its `spacing` declaration for the `body` element then these
-		* user-generated values take precedence in the CSS cascade.
-		* @link https://github.com/WordPress/gutenberg/issues/36147.
-		*/
-		$css .= 'body { margin: 0;';
-
 		/*
 		* If there are content and wide widths in theme.json, output them
 		* as custom properties on the body element so all blocks can use them.
@@ -2643,11 +2643,19 @@ class WP_Theme_JSON {
 			$content_size = static::is_safe_css_declaration( 'max-width', $content_size ) ? $content_size : 'initial';
 			$wide_size    = isset( $settings['layout']['wideSize'] ) ? $settings['layout']['wideSize'] : $settings['layout']['contentSize'];
 			$wide_size    = static::is_safe_css_declaration( 'max-width', $wide_size ) ? $wide_size : 'initial';
-			$css         .= '--wp--style--global--content-size: ' . $content_size . ';';
-			$css         .= '--wp--style--global--wide-size: ' . $wide_size . ';';
+			$css .= static::ROOT_CSS_PROPERTIES_SELECTOR . ' { --wp--style--global--content-size: ' . $content_size . ';';
+			$css .= '--wp--style--global--wide-size: ' . $wide_size . '; }';
 		}
 
-		$css .= ' }';
+		/*
+		* Reset default browser margin on the body element.
+		* This is set on the body selector **before** generating the ruleset
+		* from the `theme.json`. This is to ensure that if the `theme.json` declares
+		* `margin` in its `spacing` declaration for the `body` element then these
+		* user-generated values take precedence in the CSS cascade.
+		* @link https://github.com/WordPress/gutenberg/issues/36147.
+		*/
+		$css .= 'body { margin: 0; }';
 
 		if ( $use_root_padding ) {
 			// Top and bottom padding are applied to the outer block container.
@@ -2670,16 +2678,15 @@ class WP_Theme_JSON {
 		$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_value       = isset( $this->theme_json['styles']['spacing']['blockGap'] ) ? $this->theme_json['styles']['spacing']['blockGap'] : '0.5em';
-		$has_block_gap_support = isset( $this->theme_json['settings']['spacing']['blockGap'] );
-		if ( $has_block_gap_support ) {
+		// Block gap styles will be output unless explicitly set to `null`. See static::PROTECTED_PROPERTIES.
+		if ( isset( $this->theme_json['settings']['spacing']['blockGap'] ) ) {
 			$block_gap_value = static::get_property_value( $this->theme_json, array( 'styles', 'spacing', 'blockGap' ) );
 			$css            .= ":where(.wp-site-blocks) > * { margin-block-start: $block_gap_value; margin-block-end: 0; }";
 			$css            .= ':where(.wp-site-blocks) > :first-child:first-child { margin-block-start: 0; }';
 			$css            .= ':where(.wp-site-blocks) > :last-child:last-child { margin-block-end: 0; }';
 
 			// For backwards compatibility, ensure the legacy block gap CSS variable is still available.
-			$css .= "$selector { --wp--style--block-gap: $block_gap_value; }";
+			$css .= static::ROOT_CSS_PROPERTIES_SELECTOR . " { --wp--style--block-gap: $block_gap_value; }";
 		}
 		$css .= $this->get_layout_styles( $block_metadata );
 
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 909a67ffd2..638c33e2f2 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.6-alpha-58122';
+$wp_version = '6.6-alpha-58123';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.