There are more than 1500 themes in the WordPress site that you can search, download, and use. There is a mix of Free and Commercial themes available for download. The commercial charges may be access cost, or membership site, or free download and charge for support as per the provider. They are currently segregated with tags for different aspects of colors, columns, width, features, and subject of the theme. Each Theme available has categorized details (Description, Stats, Support, Reviews, Developers) that you can use for selection. If you do not wish to use the available themes, you can also create your own theme.
Child Theme: You can create a new look by creating a Child Theme by just changing the present Stylesheet file (style.css).
Functions File: Changing the Functions file is another way to change the look and layout of WordPress. Using Functions file, you can enable theme features (such as Sidebars, Navigation Menus, Post Thumbnails, Post Formats, Custom Headers, Custom Backgrounds and others), define functions used in several template files, or set up options menu using the file.
Template Files: WordPress defines Templates as PHP source files used to generate the pages requested by visitors, and are output as HTML. Template files are made up of HTML, PHP, and WordPress Template Tags. WordPress allows you to define separate Templates for the various aspects of your site. Templates are chosen and generated based upon the Template Hierarchy. There are 19 template files, but the basic templates (at the very minimum) consists of two files ‘style.css’ and ‘index.php’. You can, however, have more files in your created themes.
WordPress Theme Directory: http://wordpress.org/extend/themes/
WordPress Support: http://codex.wordpress.org/Main_Page
WordPress Theme Development: http://codex.wordpress.org/Theme_Development