CSS properties in textarea in the Customizer

As it happens, I was literally just doing some stuff with the customizer.

You can use the following class to get what you need..

WP_Customize_Code_Editor_Control

I’ll include it in the control ready to go.

$wp_customize->add_control( new WP_Customize_Code_Editor_Control( $wp_customize, 'favorite_html', array(
        'label' => 'Theme CSS',
        'description' => '',
        'code_type' => 'text/css',
        'section' => 'section_id',
        'settings' => 'settings_control'
    )));