wp-content/themes/customtheme/index.php

<nav class="navbar gradient-bg main-custom-nav">...
<div class="jumbotron gradient-bg margin-neg-20">
    <div class="container text-center">
        <h1>Welcome To Our Custom Site</h1>
    </div>
</div>

wp-content/themes/customtheme/style.css

.gradient-bg {
    /* Permalink - use to edit and share this gradient: http://colorzilla.com/gradient-editor/#1e5799+0,2a8ee0+31,207cca+100,7db9e8+100 */
    background: #1e5799; /* Old browsers */
    background: rgba(63,125,248,1);
    background: -moz-linear-gradient(left, rgba(63,125,248,1) 0%, rgba(105,153,242,1) 41%, rgba(53,125,233,1) 100%);
    background: -webkit-gradient(left top, right top, color-stop(0%, rgba(63,125,248,1)), color-stop(41%, rgba(105,153,242,1)), color-stop(100%, rgba(53,125,233,1)));
    background: -webkit-linear-gradient(left, rgba(63,125,248,1) 0%, rgba(105,153,242,1) 41%, rgba(53,125,233,1) 100%);
    background: -o-linear-gradient(left, rgba(63,125,248,1) 0%, rgba(105,153,242,1) 41%, rgba(53,125,233,1) 100%);
    background: -ms-linear-gradient(left, rgba(63,125,248,1) 0%, rgba(105,153,242,1) 41%, rgba(53,125,233,1) 100%);
    background: linear-gradient(to right, rgba(63,125,248,1) 0%, rgba(105,153,242,1) 41%, rgba(53,125,233,1) 100%);
    filter: progid:DXImageTransform.Microsoft.gradient( startColorstr='#3f7df8', endColorstr='#357de9', GradientType=1 );
    color: white;
}
.margin-neg-20 {
    margin-top:-20px;
}
.main-custom-nav {
    border-radius: 0;
}