newsupernova/vendor/magento/module-variable/view/adminhtml/templates/system/variable/js.phtml

<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
?>
<script>
require([
    'prototype'
], function () {

window.toggleValueElement = function(element) {
    var disabled = false;
    if (element.value == 1) {
        // use default value
        disabled = true;
    }
    $('plain_value').disabled = disabled;
    $('html_value').disabled = disabled;
    if (disabled) {
        $('plain_value').addClassName('disabled');
        $('html_value').addClassName('disabled');
    } else {
        $('plain_value').removeClassName('disabled');
        $('html_value').removeClassName('disabled');
    }
}

});
</script>