You can customize your store HTML, CSS and JS with 2 approaches.
The first one is to create an HTML widget from ADMIN > SETUP > WIDGETS. Here you can create HTML blocks that you want to be placed on your side and associate styling with it as per your requirement.
The second approach is to directly edit the code from ADMIN > SETUP > PAGES > FOOTER. Using this page, you can either write custom html/css/js code or modify existing site elements using JS/CSS. You'll find example code already in place here for your reference.
If you want us to add some custom css/js/html to your store, just drop us a mail on support@shopnix.in and we'll do it for you.