Hi Tomek,
in Tools / Custom HTML / Head<link rel=”stylesheet” href=”https://cdn.snipcart.com/themes/v3.0.7/default/snipcart.css” />
<link rel="stylesheet" href="https://cdn.snipcart.com/themes/v3.0.7/default/snipcart.css" />
<script async="async" src="https://cdn.snipcart.com/themes/v3.0.7/default/snipcart.js"></script>
in Tools / Custom CSS
You will probably need to change the z-index value of menu bar to have the shopping cart in the foreground, eg:
.container>header{z-index:9;} /*Mercury theme*/
Documentation:
https://docs.snipcart.com/v3/
