Sometimes the best way to review CSS is to use your built in browser tools. Most modern browsers have them built in. Try right-clicking an element on the page and look for something like an "Inspect Element" option. It will show you the HTML for the element you clicked as well as all of the CSS that applies to it.
You can still reach the theme's stylesheet from your Appearance → Custom Design → CSS page, click "Edit" next to "Mode: Add-on." However, note that you should not copy the entire stylesheet into your CSS editor. We found that a lot of CSS beginners were copying and pasting the entire stylesheet into Add-on mode which caused them trouble if they weren't advanced enough with CSS to know how to adjust it for a new location.