Make Mobile Nav Top Bar Sticky

[cjlangreo]

Make the mobile only Navbar sticky.

.wy-nav-top {
    position: sticky;
    top: 0;
    z-index: 100;
}

.wy-nav-top,
.wy-nav-top a {
    background-color: var(--navbar-background-color);
    color: var(--navbar-level-1-color);
}