source: multisport/1.2.3/inc/customizer.php

<?php

if ( ! function_exists( 'multisport_customize_register' ) ) :
        /**
         * Add postMessage support for site title and description for the Theme Customizer.
         *
         */
        function multisport_customize_register( $wp_customize ) {

                /**
                 * Add Footer Section
                 */
                $wp_customize->add_section(
                        'multisport_footer_section',
                        array(
                                'title'       => __( 'Footer', 'multisport' ),
                                'capability'  => 'edit_theme_options',
                        )
                );
                
                // Add Footer Copyright Text
                $wp_customize->add_setting(
                        'multisport_footer_copyright',
                        array(
                            'default'           => '',
                            'sanitize_callback' => 'sanitize_text_field',
                        )
                );

                $wp_customize->add_control( new WP_Customize_Control( $wp_customize, 'multisport_footer_copyright',
                array(
                    'label'          => __( 'Copyright Text', 'multisport' ),
                    'section'        => 'multisport_footer_section',
                    'settings'       => 'multisport_footer_copyright',
                    'type'           => 'text',
                    )
                )
                );

                /**
             * Add Animations Section
             */
            $wp_customize->add_section(
                'multisport_animations_display',
                array(
                    'title'       => __( 'Animations', 'multisport' ),
                    'capability'  => 'edit_theme_options',
                )
            );

            // Add display Animations option
            $wp_customize->add_setting(
                    'multisport_animations_display',
                    array(
                            'default'           => 1,
                            'sanitize_callback' => 'multisport_sanitize_checkbox',
                    )
            );

            $wp_customize->add_control( new WP_Customize_Control( $wp_customize,
                                'multisport_animations_display',
                                    array(
                                        'label'          => __( 'Enable Animations', 'multisport' ),
                                        'section'        => 'multisport_animations_display',
                                        'settings'       => 'multisport_animations_display',
                                        'type'           => 'checkbox',
                                    )
                                )
            );

            /**
                 * Add Slider Section
                 */
                $wp_customize->add_section(
                        'multisport_slider_section',
                        array(
                                'title'       => __( 'Slider', 'multisport' ),
                                'capability'  => 'edit_theme_options',
                        )
                );

                $wp_customize->add_setting(
                                'multisport_slider_display',
                                array(
                                                'default'           => 0,
                                                'sanitize_callback' => 'multisport_sanitize_checkbox',
                                )
                );

                $wp_customize->add_control( new WP_Customize_Control( $wp_customize, 'multisport_slider_display',
                                                                array(
                                                                        'label'          => __( 'Display Slider on a Static Front Page', 'multisport' ),
                                                                        'section'        => 'multisport_slider_section',
                                                                        'settings'       => 'multisport_slider_display',
                                                                        'type'           => 'checkbox',
                                                                )
                                                        )
                );
                
                for ($i = 1; $i <= 3; ++$i) {
                
                        $slideImageId = 'multisport_slide'.$i.'_image';
                        $defaultSliderImagePath = get_template_directory_uri().'/images/slider/'.$i.'.jpg';
                        
                        // Add Slide Background Image
                        $wp_customize->add_setting( $slideImageId,
                                array(
                                        'default' => $defaultSliderImagePath,
                                        'sanitize_callback' => 'multisport_sanitize_url'
                                )
                        );

                        $wp_customize->add_control( new WP_Customize_Image_Control( $wp_customize, $slideImageId,
                                        array(
                                                'label'          => sprintf( esc_html__( 'Slide #%s Image', 'multisport' ), $i ),
                                                'section'        => 'multisport_slider_section',
                                                'settings'   => $slideImageId,
                                        ) 
                                )
                        );
                }
        }
endif; // multisport_customize_register
add_action( 'customize_register', 'multisport_customize_register' );

if ( ! function_exists( 'multisport_sanitize_checkbox' ) ) :
        /**
         * Sanitization callback for 'checkbox' type controls. This callback sanitizes `$checked`
         * as a boolean value, either TRUE or FALSE.
         *
         * @param bool $checked Whether the checkbox is checked.
         * @return bool Whether the checkbox is checked.
         */
        function multisport_sanitize_checkbox( $checked ) {
                // Boolean check.
                return ( ( isset( $checked ) && true == $checked ) ? true : false );
        }
endif; // multisport_sanitize_checkbox

if ( ! function_exists( 'multisport_sanitize_url' ) ) :

        function multisport_sanitize_url( $url ) {
                return esc_url_raw( $url );
        }

endif; // multisport_sanitize_url