向下滚动并使用 Java 脚本修复标头

我希望当我向下滚动时标题会有背景，但在顶部标题将是透明的，就像这个链接一样，我可以使用引导程序将该标题固定在顶部，但我希望当我向下滚动时它会得到修复。

这是我的网站布局在此处输入链接说明

HTML 代码是' 切换导航 正中电

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    <!-- Cover Page Text Starts Here -->
    <section class="first firstheading">
    <Div class="psr">
        <div class="container-fluid">
            <h2 id="social">Gain International Experience</h2>
            <p>We offer abroad internship programme for students and recent gradudates</p>
            </div>
            <ul>
                <li>LONDON</li>
                <li>DUBAI</li>
                <li>INDIA</li>
                <li>CHINA</li>
            </ul>
        </div>
    </section>
    <!-- End of Cover Page -->
    <!-- Slider Starts Here -->
    <section class="second-sc">
        <div id="carousel-example-generic" class="carousel slide" data-ride="carousel" data-interval="false">
          <!-- Indicators -->
          <div class="container-fluid second-menu">
              <ol class="carousel-indicators">
                <li data-target="#carousel-example-generic" data-slide-to="0" class="second-menu-active">LONDON</li>
                <li data-target="#carousel-example-generic" data-slide-to="1">DUBAI</li>
                <li data-target="#carousel-example-generic" data-slide-to="2">MUMBAI</li>
                <li data-target="#carousel-example-generic" data-slide-to="3">BANGALORE</li>
                <li data-target="#carousel-example-generic" data-slide-to="4">BEIGING</li>
              </ol>
            </div>
            <div class="container-fluid dots">
              <ol class="carousel-indicators">
                <li data-target="#carousel-example-generic" data-slide-to="0" class="dots-active"></li>
                <li data-target="#carousel-example-generic" data-slide-to="1"></li>
                <li data-target="#carousel-example-generic" data-slide-to="2"></li>
                <li data-target="#carousel-example-generic" data-slide-to="3"></li>
                <li data-target="#carousel-example-generic" data-slide-to="4"></li>
              </ol>
            </div>
          <!-- Wrapper for slides -->
          <div class="carousel-inner" role="listbox">
            <div class="item active">
                <div class="second">
                  <div class="second-menu">
                    <h2>DUBAI INTERSHIP PROGRAMME</h2>
                    <p>Dubai is a great desitnation if you’re looking for corporate and real estate experience. We work with over 60 companies in Dubai</p>
                    <div class="second-dugme"><a href="">FIND OUT MORE</a></div>
                </div>
                </div>
            </div>
            <div class="item ">
             <div class="second">
                  <div class="second-menu">
                    <h2>DUBAI INTERSHIP PROGRAMME</h2>
                    <p>Dubai is a great desitnation if you’re looking for corporate and real estate experience. We work with over 60 companies in Dubai</p>
                    <div class="second-dugme"><a href="">FIND OUT MORE</a></div>
                </div>
                </div>
            </div>
             <div class="item ">
             <div class="second">
                  <div class="second-menu">
                    <h2>DUBAI INTERSHIP PROGRAMME</h2>
                    <p>Dubai is a great desitnation if you’re looking for corporate and real estate experience. We work with over 60 companies in Dubai</p>
                    <div class="second-dugme"><a href="">FIND OUT MORE</a></div>
                </div>
                </div>
            </div>
             <div class="item ">
             <div class="second">
                  <div class="second-menu">
                    <h2>DUBAI INTERSHIP PROGRAMME</h2>
                    <p>Dubai is a great desitnation if you’re looking for corporate and real estate experience. We work with over 60 companies in Dubai</p>
                    <div class="second-dugme"><a href="">FIND OUT MORE</a></div>
                </div>
                </div>
            </div>
             <div class="item ">
             <div class="second">
                  <div class="second-menu">
                    <h2>DUBAI INTERSHIP PROGRAMME</h2>
                    <p>Dubai is a great desitnation if you’re looking for corporate and real estate experience. We work with over 60 companies in Dubai</p>
                    <div class="second-dugme"><a href="">FIND OUT MORE</a></div>
                </div>
                </div>
            </div>
          </div>
        </div>
    </section>`

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   var sT = $(this).scrollTop();
    if (sT >= 300) {
        $('.overlay').addClass('cambio')
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