Вы присвоили фон navbar-nav? Вы можете назначить цвет, чтобы быть уверенным в этом
рабочий фрагмент ниже:
.restOfThePage {
margin-top: 100px;
}
.navbar-nav {
background-color: #fff;
}
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.4.1/css/bootstrap.min.css">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.4.1/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.16.0/umd/popper.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.4.1/js/bootstrap.min.js"></script>
<nav class="navbar navbar-light navbar-expand-sm fixed-top">
<div class="container">
<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#Navbar">
<i class="navbar-toggler-icon"></i>
</button>
<a class="mr-auto navbar-brand nh myfont" href=".\index.html"> <img src="images/logo.png" width=6 0px height=7 0px> Sidhant Aggarwal</a>
<div class="collapse navbar-collapse" id="Navbar">
<ul class="navbar-nav mr-auto nl">
<li class="nav-item mrn"> <a class="nav-link" href="#about"> About</a></li>
<li class="nav-item mrn"> <a class="nav-link" href="#"> Resume</a></li>
<li class="nav-item mrn"> <a class="nav-link" href=".\contactus.html"> Certifications</a></li>
</ul>
</div>
<ul class="navbar-nav ml-auto nav-flex-icons flex-row">
<li class="nav-item"> <a class="btn btn-social-icon" href="http://www.linkedin.com/in/"><i class = "fa fa-linkedin fa-xs"></i></a></li>
<li class="nav-item"><a class="btn btn-social-icon" href="mailto:"><i class = "fa fa-github fa-xs"></i></a></li>
<li class="nav-item"><a class="btn btn-social-icon" href="mailto:"><i class = "fa fa-envelope fa-xs"></i></a></li>
</ul>
</div>
</nav>
<div class='restOfThePage'>
<h1>Other content</h1>
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</div>