There are 2 locations for you to customize the look of your store:
1) Basic Configurationpage: Here you can change the name of the store, the number of items to display per page, assign header and footer images, and set a title and a description to the store as well as a number of other configuration options.
2) Colors and background: Here you can change all of the font styles and colors for your store (titles, primary and secondary text, buttons, links, etc.) as well as background colors (exterior and interior background, menus, separation lines, etc.) You can even set images as a backgrounds.
The best suggestion is to explore the options and try them out while you refresh your store page to see how it looks until you're happy with the way everything looks.