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' );

/**
 * @param $wp_customize WP_Customize_Manager
 */
function twentyfifteen_customized_setting_blog_title_color( $wp_customize ) {
	$wp_customize->add_setting( 'blog_title_color', array(
		'default' => '#4f998d',
		'sanitize_callback' => 'sanitize_hex_color',
	) );
	$wp_customize->add_control( new WP_Customize_Color_Control( $wp_customize, 'blog_title_color', array(
		'label'   => __( 'Blog Title Color', 'twentyfifteen-customized' ),
		'section' => 'custom_css',
	) ) );
}
add_action( 'customize_register', 'twentyfifteen_customized_setting_blog_title_color' );

/**
 * @param $wp_customize WP_Customize_Manager
 */
function twentyfifteen_customized_setting_header_background_image( $wp_customize ) {
	$wp_customize->add_setting( 'blog_title_bg_image', array(
		'type' => 'theme_mod',
	) );
	$wp_customize->add_control( new WP_Customize_Media_Control( $wp_customize, 'blog_title_bg_image', array(
		'label' => __( 'Blog Title Background Image', 'twentyfifteen-customized' ),
		'section' => 'custom_css',
		'mime_type' => 'image',
	) ) );
}
add_action( 'customize_register', 'twentyfifteen_customized_setting_header_background_image' );

function twentyfifteen_customized_custom_css_output() {
	?>
	<style type="text/css" id="custom-theme-css">
		.site-title {
			background-image: url(<?php echo wp_get_attachment_image_url( get_theme_mod( 'blog_title_bg_image', '' ) ); ?>);
		}
	    .site-title a {
		    color: <?php echo get_theme_mod( 'blog_title_color', '#4f998d' ) ?>;
	    }
		<?php echo get_theme_mod( 'custom_theme_css', '' ); ?>
	</style>
	<?php
}
add_action( 'wp_head', 'twentyfifteen_customized_custom_css_output');

style.css

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