<?php   
$wp_customize->add_setting(
    'menu_distance_between',
    array(
        'default'   =>  themesflat_customize_default('menu_distance_between'),
        'sanitize_callback' => 'esc_attr',
    )
);
$wp_customize->add_control( new themesflat_Slide_Control( $wp_customize,
    'menu_distance_between',
        array(
            'type'      =>  'slide-control',
            'section'   =>  'section_navigation',
            'label'     =>  'Distance Between (px)',
            'priority'  => 1,
            'input_attrs' => array(
                'min' => 0,
                'max' => 100,
                'step' => 1,
            ),
        )

    )
);