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$wp_customize->add_control( 'custom_theme_css', array(
  'type' => 'date',
  'priority' => 10, // Within the section.
  'section' => 'colors', // Required, core or custom.
  'label' => __( 'Date' ),
  'description' => __( 'This is a date control with a red border.' ),
  'input_attrs' => array(
    'class' => 'my-custom-class-for-js',
    'style' => 'border: 1px solid #900',
    'placeholder' => __( 'mm/dd/yyyy' ),
  ),
  'active_callback' => 'is_front_page',
  'section' => 'custom_css',
));

$wp_customize->add_control( 'custom_theme_css', array(
  'label' => __( 'Custom Theme CSS' ),
  'type' => 'textarea',
  'section' => 'custom_css',
  ));

$wp_customize->add_control( 'custom_theme_css', array(
  'label' => __( 'Change Opacity' ),
  'type' => 'css',
  'opacity' => ''//some variable that gets user input for opacity value
  'section' => 'color',
   ));

$wp_customize->add_control( 'custom_theme_css', array(
  'label' => __( 'Set CSS Opacity Value' ),
  'type' => 'text',
  'section' => 'custom_css',
) );

$wp_customize->add_control(
   'my_control',
    array(
        'label'    => __( 'Opacity Control', 'mytheme' ),
        'section'  => 'my_section',
        'settings' => 'my_setting',
        'type'     => 'text',
    )
);

<head>
  <style>
    *{
      opacity: <?php echo get_theme_mod( 'my_control', '0' ); ?>
    }
  </style>
</head>