Mở file functions.php trong child theme của bạn (nếu chưa có child theme, bạn nên tạo child theme để tránh mất thay đổi khi update theme). Thêm đoạn code sau:

// Hướng dẫn tạo hiệu ứng "Xem thêm / Thu gọn" cho phần mô tả sản phẩm

function webdy_readmore_flatsome(){
    ?>
    <style>
        .single-product div#tab-description {
            overflow: hidden;
            position: relative;
            padding-bottom: 25px;
        }
        .fix_height{
            max-height: 800px;
            overflow: hidden;
            position: relative;
        }
        .single-product .tab-panels div#tab-description.panel:not(.active) {
            height: 0 !important;
        }
        .webdy_readmore_flatsome {
            text-align: center;
            cursor: pointer;
            position: absolute;
            z-index: 10;
            bottom: 0;
            width: 100%;
            background: #fff;
        }
        .webdy_readmore_flatsome:before {
            height: 55px;
            margin-top: -45px;
            content: "";
            background: -moz-linear-gradient(top, rgba(255,255,255,0) 0%, rgba(255,255,255,1) 100%);
            background: -webkit-linear-gradient(top, rgba(255,255,255,0) 0%,rgba(255,255,255,1) 100%);
            background: linear-gradient(to bottom, rgba(255,255,255,0) 0%,rgba(255,255,255,1) 100%);
            filter: progid:DXImageTransform.Microsoft.gradient( startColorstr='#ffffff00', endColorstr='#ffffff',GradientType=0 );
            display: block;
        }
        .webdy_readmore_flatsome a {
            color: #318A00;
            display: block;
        }
        .webdy_readmore_flatsome a:after {
            content: '';
            width: 0;
            right: 0;
            border-top: 6px solid #318A00;
            border-left: 6px solid transparent;
            border-right: 6px solid transparent;
            display: inline-block;
            vertical-align: middle;
            margin: -2px 0 0 5px;
        }      border-left: 6px solid transparent;
            border-right: 6px solid transparent;
        .webdy_readmore_flatsome_less a:after {
            border-top: 0;
      
            border-bottom: 6px solid #318A00;
        }
        .webdy_readmore_flatsome_less:before {
            display: none;
        }
    </style>
    <script>
        (function($){
            $(document).ready(function(){
                $(window).on('load', function(){
                    if($('.single-product div#tab-description').length > 0){
                        let wrap = $('.single-product div#tab-description');
                        let current_height = wrap.height();
                        let your_height = 2500;
                        if(current_height > your_height){
                            wrap.addClass('fix_height');
                            wrap.append(function(){
                                return '<div class="webdy_readmore_flatsome webdy_readmore_flatsome_more"><a title="Xem thêm" href="javascript:void(0);">Xem thêm</a></div>';
                            });
                            wrap.append(function(){
                                return '<div class="webdy_readmore_flatsome webdy_readmore_flatsome_less" style="display: none;"><a title="Xem thêm" href="javascript:void(0);">Thu gọn</a></div>';
                            });
                            $('body').on('click','.webdy_readmore_flatsome_more', function(){
                                wrap.removeClass('fix_height');
                                $('body .webdy_readmore_flatsome_more').hide();
                                $('body .webdy_readmore_flatsome_less').show();
                            });
                            $('body').on('click','.webdy_readmore_flatsome_less', function(){
                                wrap.addClass('fix_height');
                                $('body .webdy_readmore_flatsome_less').hide();
                                $('body .webdy_readmore_flatsome_more').show();
                            });
                        }
                    }
                });
            });
        })(jQuery);
    </script>
    <?php
}