magento-test/app/code/Magento/Theme/view/adminhtml/templates/tabs/css.phtml

<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
/**
 * @var $block \Magento\Theme\Block\Adminhtml\System\Design\Theme\Edit\Tab\Css
 * @var \Magento\Framework\View\Helper\SecureHtmlRenderer $secureRenderer
 */
?>

<?= $block->getFormHtml() ?>

<?php $scriptString = <<<script

require([
    "jquery",
    "Magento_Ui/js/modal/alert",
    "jquery/file-uploader",
    "mage/translate"
], function($, alert){

    $( '#css_file_uploader' ).fileupload({
        dataType: 'json',
        replaceFileInput: false,
        url : '{$block->escapeJs($block->getUrl('*/system_design_theme/uploadcss'))}',
        acceptFileTypes: /(.|\/)(css)$/i,

        /**
         * Add data
         * @param e
         * @param data
         */
        add: function (e, data) {
            var uploadButton = $('#css_uploader_button');
            /** Unbind click event on file change */
            uploadButton.off('click');
            uploadButton.prop('disabled', false);

            uploadButton.on('click', function () {
                $('#messages').html('');
                $(this).attr('disabled', 'disabled');
                data.submit();
            });
        },

        /**
         * On done event
         * @param e
         * @param data
         */
        done: function (e, data) {
            var contentArea = $('#custom_css_content');
            $(this).val('');

            $('#css_uploader_button').attr('disabled', 'disabled');

            if (!data.result.error) {
                contentArea.trigger('focusin');
                contentArea.val(data.result.content);
                contentArea.trigger('focusout');
            }
        },

        /**
         * Fail event
         * @param e
         * @param data
         */
        fail: function(e, data) {
            $(this).val('');
            alert({
                content: $.mage.__("We don't recognize this file extension.")
            });
        }
    });

    $(document).on('beforeSubmit', function() {
        $('#css_file_uploader').val('');
    });

});

script;
?>
<?= /* @noEscape */ $secureRenderer->renderTag('script', [], $scriptString, false) ?>