twentyfifteen-customized

functions.php

<?php

function twentyfifteen_parent_enqueue_styles() {
	wp_enqueue_style( 'parent-style', get_template_directory_uri() . '/style.css' );
}
add_action( 'wp_enqueue_scripts', 'twentyfifteen_parent_enqueue_styles' );

/**
 * @param $wp_customize WP_Customize_Manager
 */
function twentyfifteen_customized_setting_custom_css( $wp_customize ) {
	$wp_customize->add_setting( 'custom_theme_css', array(
		'type' => 'theme_mod',
	) );
	$wp_customize->add_control( 'custom_theme_css', array(
		'label' => __( 'Custom Theme CSS', 'twentyfifteen-customized' ),
		'type' => 'textarea',
		'section' => 'custom_css',
	) );
	$wp_customize->add_section( 'custom_css', array(
		'title' => __( 'Custom CSS', 'twentyfifteen-customized' ),
		'description' => __( 'Add custom CSS here', 'twentyfifteen-customized' ),
		'priority' => 160,
		'capability' => 'edit_theme_options',
	) );
}
add_action( 'customize_register', 'twentyfifteen_customized_setting_custom_css' );

function twentyfifteen_customized_custom_css_output() {
	echo '<style type="text/css" id="custom-theme-css">' .
	        get_theme_mod( 'custom_theme_css', '' ) .
	     '</style>';
}
add_action( 'wp_head', 'twentyfifteen_customized_custom_css_output');

style.css

/**
Theme Name: TwentyFifteen Customized
Template: twentyfifteen
*/