You've just purchased the Custom CSS upgrade. Your theme looks exactly the same as before and you now have an empty box in the CSS Editor. What next?
The CSS Editor does not let you edit any particular theme stylesheet; rather, it keeps the theme intact and lets you edit a brand new stylesheet. Because your custom CSS is loaded after the theme's CSS, your style rules can override or augment the existing rules.
You can use CSS to improve on any existing theme; you can define your own classes for use in posts; you can select the Sandbox theme and build on one of the Minimalist layouts or opt for no stylesheet and do it all yourself.
CSS is not hard to learn and there is a lot you can do with it at any level of skill.