github.com/shohhei1126/hugo@v0.42.2-0.20180623210752-3d5928889ad7/examples/blog/static/css/custom.css

body {
    margin-top: 75px; /* 100px is double the height of the navbar - I made it a big larger for some more space - keep it at 50px at least if you want to use the fixed top nav */
}

footer {
    margin: 50px 0;
}