Drupal 7 - get/set theme value

<?php
    // get setting from theme setting
    $settings      = variable_get ( 'theme_key_settings' );

    $last_field_key = $settings['field_key'];

    // increment field key by 1
    $new_field_key; = $last_field_key++;

    // set new setting to theme
    $settings['field_key'] = $new_field_key;
    variable_set ( 'theme_key_settings', $settings );