Index: src/wp-admin/css/customize-controls.css
===================================================================
--- src/wp-admin/css/customize-controls.css	(revision 36219)
+++ src/wp-admin/css/customize-controls.css	(working copy)
@@ -533,6 +533,13 @@
 	margin-bottom: 5px;
 }
 
+.customize-control-description a.external-link:after {
+    font: 16px/11px dashicons;
+    content: "\f310";
+    top: 3px;
+    position: relative;
+}
+
 .customize-control-color .color-picker,
 .customize-control-upload div {
 	line-height: 28px;
Index: src/wp-includes/class-wp-customize-control.php
===================================================================
--- src/wp-includes/class-wp-customize-control.php	(revision 36219)
+++ src/wp-includes/class-wp-customize-control.php	(working copy)
@@ -423,7 +423,7 @@
 	 *
 	 * Allows the content to be overriden without having to rewrite the wrapper in $this->render().
 	 *
-	 * Supports basic input types `text`, `checkbox`, `textarea`, `radio`, `select` and `dropdown-pages`.
+	 * Supports basic input types `none`, `text`, `checkbox`, `textarea`, `radio`, `select` and `dropdown-pages`.
 	 * Additional input types such as `email`, `url`, `number`, `hidden` and `date` are supported implicitly.
 	 *
 	 * Control content can alternately be rendered in JS. See {@see WP_Customize_Control::print_template()}.
@@ -432,6 +432,14 @@
 	 */
 	protected function render_content() {
 		switch( $this->type ) {
+			case 'none':
+					if ( ! empty( $this->label ) ) : ?>
+						<span class="customize-control-title"><?php echo esc_html( $this->label ); ?></span>
+					<?php endif;
+					if ( ! empty( $this->description ) ) : ?>
+						<span class="description customize-control-description"><?php echo $this->description; ?></span>
+					<?php endif;
+				break;
 			case 'checkbox':
 				?>
 				<label>
Index: src/wp-includes/class-wp-customize-manager.php
===================================================================
--- src/wp-includes/class-wp-customize-manager.php	(revision 36219)
+++ src/wp-includes/class-wp-customize-manager.php	(working copy)
@@ -2075,6 +2075,34 @@
 				'type'       => 'dropdown-pages',
 			) );
 		}
+
+		/* Custom CSS */
+
+		$this->add_section( 'custom_css', array(
+			'title'    => __( 'Custom CSS' ),
+			'priority' => 140,
+		) );
+
+		$this->add_setting( 'wp_custom_css' , array(
+			'type'      => 'theme_mod',
+			'transport' => 'postMessage',
+		) );
+
+		$this->add_control( 'wp_custom_css', array(
+			'label'       => __( 'Custom CSS' ),
+			'description' => __( 'CSS allows you to customize the appearance and layout of your site with code. Each theme has its own set of CSS styles, which this option overrides on a per-theme basis. <a href="https://codex.wordpress.org/Know_Your_Sources#CSS" class="external-link">Learn more about CSS <span class="screen-reader-text">(link opens in a new window)</span></a>.' ),
+			'type'        => 'textarea',
+			'section'     => 'custom_css',
+		) );
+
+		if ( ! is_multisite() ) {
+			$this->add_setting( 'wp_custom_css_more', array() );
+			$this->add_control( 'wp_custom_css_more', array(
+				'type' => 'none',
+				'description' => __( 'Enjoy writing custom CSS? <a href="https://developer.wordpress.org/themes/advanced-topics/child-themes/" class="external-link">Take your customizations to the next level with a child theme <span class="screen-reader-text">(link opens in a new window)</span></a>.' ),
+				'section'     => 'custom_css',
+			) );
+		}
 	}
 
 	/**
Index: src/wp-includes/default-filters.php
===================================================================
--- src/wp-includes/default-filters.php	(revision 36219)
+++ src/wp-includes/default-filters.php	(working copy)
@@ -236,6 +236,7 @@
 add_action( 'wp_head',             'wp_generator'                           );
 add_action( 'wp_head',             'rel_canonical'                          );
 add_action( 'wp_head',             'wp_shortlink_wp_head',            10, 0 );
+add_action( 'wp_head',             'wp_custom_css_cb',                11    );
 add_action( 'wp_head',             'wp_site_icon',                    99    );
 add_action( 'wp_footer',           'wp_print_footer_scripts',         20    );
 add_action( 'template_redirect',   'wp_shortlink_header',             11, 0 );
Index: src/wp-includes/js/customize-preview.js
===================================================================
--- src/wp-includes/js/customize-preview.js	(revision 36219)
+++ src/wp-includes/js/customize-preview.js	(working copy)
@@ -169,6 +169,12 @@
 			});
 		});
 
+		api( 'wp_custom_css', function( value ) {
+			value.bind( function( to ) {
+				$( '#wp-custom-css' ).html( to );
+			} );
+		} );
+
 		api.trigger( 'preview-ready' );
 	});
 
Index: src/wp-includes/theme.php
===================================================================
--- src/wp-includes/theme.php	(revision 36219)
+++ src/wp-includes/theme.php	(working copy)
@@ -1396,7 +1396,24 @@
 <?php
 }
 
+
 /**
+ * Render custom CSS.
+ *
+ * @since 4.5
+ */
+function wp_custom_css_cb() {
+	$styles = get_theme_mod( 'wp_custom_css' );
+	if ( $styles ) :
+?>
+	<style type="text/css" id="wp-custom-css">
+		<?php echo $styles; ?>
+	</style>
+<?php
+	endif;
+}
+
+/**
  * Add callback for custom TinyMCE editor stylesheets.
  *
  * The parameter $stylesheet is the name of the stylesheet, relative to