CSS (Cascading Style Sheets) is used to control the appearance of just about everything on the internet. Custom CSS gives you fine level control over the styling on the page, allowing you to control the coloring, spacing, sizing, and lots more! Custom CSS can be added under Theme → UI Elements → Custom CSS.
Using custom CSS for styling requires a little technical knowledge of CSS; if you're new to CSS then a short course like this might help you get started. If you get stuck, just to ask us in chat or at email@example.com.