The main CSS style is style.css, which is located in the root folder. You can modify it, however, if you ever upgrade to a newer version of the theme, all your changes to style.css will be replaced with the new version. Probably not what you want.
So instead of modifying the style.css file directly, you can modify it in a safer way 2 ways:
CSS setting in Theme Options
You can modify the CSS that affects globally all the pages in the site in the Theme Options. In the General section, there is a setting for CSS. You can put in your custom style in there. If you want to override something in style.css, then simply enter the same exact selector as used in style.css, and then add or modify any of the properties.
CSS Custom Field
You can add or override any CSS on a page-by-page (or post-by-post) basis by using the “css” Custom Field (it’s case sensitive). At the bottom of a new Page or Post, add a Custom Field called “css” (case sensitive). For the value, type in your style. This style will be added ONLY to the current page, and overrides all other non-inline styles.