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<?php
add_action( 'customize_register', 'prefix_remove_css_section', 15 );
/**
 * Remove the additional CSS section, introduced in 4.7, from the Customizer.
 * @param $wp_customize WP_Customize_Manager
 */
function prefix_remove_css_section( $wp_customize ) {
	$wp_customize->remove_section( 'custom_css' );
}