Customizer Function

customizer.php

<?php

function extend_customizer() {
    global $wp_customize;

    // Add Panel
    $wp_customize->add_panel('example', array(
        'title'       => 'Example Panel',
        'description' => 'Description',
        'priority'    => 80
    ));

    // Add Section
    $wp_customize->add_section('example_section', array(
        'title'       => 'Section',
        'description' => 'Description',
        'priority'    => 10,
        'panel'       => 'example' // optional
    ));

    // Add Setting
    $wp_customize->add_setting('example_setting', array(
        'default'   => 'Default Value',
        'type'      => 'theme_mod'
    ));

    // Add Control
    $wp_customize->add_control('example_control', array(
        'label'        => 'Example Control',
        'description'  => 'Description',
        'settings'     => 'example_setting',
        'section'      => 'example_section',
        'type'         => 'text'
    ));

    // Add Custom Control
    $wp_customize->add_control(
        new WP_Customize_Color_Control(
            $wp_customize,
            'example_control',
            array(
                'label'       => 'Example Control',
               'description' => 'Description',
               'settings'    => 'example_setting',
                   'section'     => 'example_section'
            )
        )
    );
}

add_action('customize_register', 'extend_customizer');