To add custom CSS rules to your storefront, head over to your dashboard and click on Settings in the menu. On the settings page, click the Store Settings tab, and scroll down to the box called Advanced Styling.
Add your CSS rules to the input titled Custom CSS and then click save at the bottom of the page.
To see your new rules in action, click on the Storefront button in the menu.
If your CSS rules don't immediately show up, try emptying the cache on your browser, or try a different browser.
Here is an example CSS rule, which boosts the font size on the featured product title:
.featured-product-name {
font-size: 36px;
}