Homepage
Besides adding a lot of custom CSS to adjust the colors, position, and size of certain elements, I also modified several .liquid template files to improve the UI. For example, the green call-to-action buttons just above the footer are a custom element that the client requested.
Product Page
The product page is fairly standard, though it was also set up with warranty and customer reviews plugins.
Checkout
The client had the unique requirement that each battery purchased from the store had to be assigned a desposit fee acting as collateral for the specialized 'core' container in which the product would be shipped. The customer, upon sending back this container, would be refunded the full amount. Since Shopify had no built-in methods to apply an additional, refundable fee of that sort, I set up this feature using the Order & Product fees plugin.