How to create a fixed navigation on Shopping Cart Page?

Hi Guys!!

It's me again, thank you so much for the community's help so far.

I'm curious how to code the navigation bar on the shopping cart page to be fixed/sticky.

Right now it looks all jumbled and not right, I've attached an image below.

The website URL is: https://penguin-roadrunner-4c8m.squarespace.com/

Password: Skyfall77

add to Home > Design > Custom CSS

body#cart header.Header.Header--top {
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    z-index: 999;
}

54 minutes ago, tuanphan said:

body#cart header.Header.Header--top { position: fixed; top: 0; left: 0; right: 0; z-index: 999; }

Hmm I did that and it distorted further. It also has blue leaking through.