I would like to implement the nav bar shadow on this site. I felt it was cool.

I looked at CSS, but I'm having trouble implementing it.
How can I get a shadow on the NAV bar like this site?

You could implement the bootstrap NAV bar yourself.

@media (max-width: 960px)
<style>
.layout-wrapper .common-header [data-v-76b749f6] {
    position: fixed;
    top: 0;
    height: 96px;
}
<style>
.layout-wrapper .common-header [data-v-76b749f6] {
    height: 74px;
    border: none;
}
<style>
.header [data-v-a5f295d0] {
    position: relative;
    z-index: 2;
    width: 100%;
    height: 56px;
    background-color: #fff;
    box-sizing: border-box;
    box-shadow: 0 0 6px #aaa;