fix top navigation bar

As you can see from the css-style files of that side,
that navigation bar has a css class of “navbar-fixed-top” which makes it “fixed on top”.

That is a built-in theme functionality for that other website,
you may want to change your own css file to have the same styling starting with:

.header {
    position: fixed;
    z-index: 9999;
    right: 0;
    left: 0;
}
#container {
    position: relative;
    display: block;
}

Then you may modify some other settings, etc…