The Navbar of bootstrap 4 is not transparent to me, try placing a class called to the navbar called <nav class="my-navbar">
And remove the color in the CSS in this way:
.my-navbar{
border:0px;
border-radius: 0px;
background:none;
}
and the color that comes by default bootstrap 4 is not removed.
Navbar code
<div class="col-12 d-flex justify-content-center">
<nav class="navbar navbar-expand-lg navbar-light bg-light my-navbar">
<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarNav" aria-controls="navbarNav" aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon"></span>
</button>
<div class="collapse navbar-collapse" id="navbarNav">
<ul class="navbar-nav">
<li class="nav-item active">
<a class="nav-link " href="#">Home</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">My gifts</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">Contact</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">Register</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#"><button type="button" class="btn btn-info">Login</button>
</a>
</li>
</ul>
</div>
</nav>
</div>