Making nav banner always visible

Site URL: https://flatworm-sawfish-bgzs.squarespace.com/

Password :: Flathead522

This is my current code for my navigation banner, I am looking to make it so as you scroll through the pages it always still visible no matter how far down you are.

header#header {
    background-color: white !important;
}
header#header .header-nav-item a {
    color: black;
}
header#header .burger-inner>div {
    background-color: black;
}