Smart Cart is a powerful platform designed to be built on. For radical customizations, we have a solution in place called Custom Templates. However, for most modifications to Smart Cart, Custom Templates are not required. Instead, you can modify Smart Cart by targeting specific HTML elements in the cart and modifying the CSS.
For example, you can use Chrome or other web browsers to understand and inspect the HTML elements that compose the cart. You can see which default styles apply to each element. Using CSS in your theme's header section, you can target these Smart Cart elements and override the styles applied to each. By doing so you can have full control over both the layout and look and feel of the cart without having to create a Custom Template.