我一直在尝试保持navbar，但是当我滚动时，navbar在通过标题部分时就会消失。我将标头部分的高度指定为100VH。我需要navbar改变颜色，这样它就可以在白色背景的其他部分上看到。

 <header class="header">
        <nav class="navbar navbar-expand-lg navbar-custom fixed-top">
            <div class="container">
                <a href="#" class="navbar-brand">Sharvanand</a>
                <button class="navbar-toggler" data-toggle="collapse" data- 
                   target="#navbarMenu">
                   <!--<span class="navbar-toggler-icon custom-toggler"></span>-->
                   <i class="fas fa-bars fa-lg"></i>
                    </button>
                        <div class="navbar-collapse collapse" id="navbarMenu">
                             <ul class="navbar-nav ml-auto">
                            <li class="nav-item active "><a href="#about-me" class="nav-link scroll">About Me</a></li>
                            <li class="nav-item"><a href="#projects" class="nav-link scroll">Projects</a></li>
                             <li class="nav-item "><a href="#skills" class="nav-link scroll">Skills</a></li>
                            <li class="nav-item"><a href="#education"class="nav-link scroll">Education</a></li>
                             </ul>
                             </div>
                             </div>
                            </nav>
                             <div class="main-box">
                             <img src="img/shaggy.jpg" alt="sharvanand">
                            <h3>Front End Developer</h3>
                         <div class="text-center">
                     <a href="https://github.com/Sharvanand" target="blank" class="btn 
                 custom-btn">Visit GitHub</a>
                <a href="#" class="btn custom-btn">Download Resume</a>
            </div>
        </div>
   </header>
 
This is the styling css part
.header {
  position: relative;
  width: 100%;
  height:100vh;
  background-image: linear-gradient(
    to right bottom,
    rgba(1, 9, 49, 0.801),
    rgba(90, 105, 148, 0.801)
  );
  clip-path: polygon(0 0, 100% 0, 100% 85%, 0% 100%);
}
/* change the brand and text color */
.navbar-custom .navbar-brand,
.navbar-custom .navbar-text {
  color: rgba(205, 206, 247, 0.8);
  text-transform: uppercase;
  letter-spacing: 0.3rem;
 
}
/* change the link color */
.navbar-custom .navbar-nav .nav-link {
    color: rgba(255, 255, 255, 0.5); 
    letter-spacing: 0.2rem;
    text-transform: uppercase;
    margin-right: 10px;
  
 
}
/* change the color of active or hovered links */
.navbar-custom .nav-item.active .nav-link,
.navbar-custom .nav-item:hover .nav-link {
  color: #ffffff;
}