If you have Designer permissions you can change things like the overall colour scheme, layout and fonts used on the site. Most administrators won't have this permission though - you can tell if you do because when you're on the Dashboard page you'll see an Appearance option in the left hand menu.

Even if you don't have Designer permissions you can still make styling changes on a page basis. For instance, by clicking the Page Settings button on the page you're on you can change the banner image/slideshow for that page, and by Editing the Styles in a particular block you can change that block's background and foreground colours.