Theme settings not getting saved

Question

I created custom settings for my theme for storing email, fb url, twitter url and g+ url. Everything is coming properly but the settings are not getting saved. I have no idea whats wrong. All the code and screenshots are below.

.info file content

settings[z_email] = mail@zed.com
settings[z_facebook] = fb/url
settings[z_twitter] = twitter/url
settings[z_gplus] = gplus/url

theme-settings.php file content

function z_form_system_theme_settings_alter(&$form, $form_state) {
  $form['z_email'] = array(
    '#type'          => 'textfield',
    '#title'         => t('Email'),
    '#default_value' => theme_get_setting('mail@zealind.com'),
    '#description'   => t("Global Email Id for ZealInd site."),
  );

  $form['z_facebook'] = array(
    '#type'          => 'textfield',
    '#title'         => t('Facebook'),
    '#default_value' => theme_get_setting(''),
    '#description'   => t("Facebook Url."),
  );

  $form['z_twitter'] = array(
    '#type'          => 'textfield',
    '#title'         => t('Twitter'),
    '#default_value' => theme_get_setting(''),
    '#description'   => t("Twitter Url."),
  );

  $form['z_gplus'] = array(
    '#type'          => 'textfield',
    '#title'         => t('Google+'),
    '#default_value' => theme_get_setting(''),
    '#description'   => t("Google+ Url."),
  );



}

The theme setting page: