Navbar

If you're using Bootstrap, you can replace Bootstrap CSS with your favorite Bootswatch Theme.  There are 25 free themes to choose from.

https://www.bootstrapcdn.com/bootswatch/

Example with Sketchy Theme:

<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bootswatch@4.5.2/dist/sketchy/bootstrap.min.css" integrity="sha384-RxqHG2ilm4r6aFRpGmBbGTjsqwfqHOKy1ArsMhHusnRO47jcGqpIQqlQK/kmGy9R" crossorigin="anonymous">

You can change navbar styles with built-in Bootstrap color classes.

https://getbootstrap.com/docs/4.0/components/navbar/#color-schemes

<nav class="navbar navbar-dark bg-dark">
  <!-- Navbar Dark -->
</nav>

<nav class="navbar navbar-dark bg-primary">
  <!-- Navbar Dark with Primary background color -->
</nav>

<nav class="navbar navbar-light" style="background-color: #e3f2fd;">
  <!-- Navbar light with custom background-color -->
</nav>

Or you can create custom navbar styles in your custom stylesheet, below Bootstrap CSS.

.navbar-nav > li > a {
    color: green;
}

.navbar-nav > li > a:hover,
.navbar-nav > li > a:active,  
.navbar-nav > li > a:focus {
    color: tomato;
}