To build a WordPress theme, you need at least two files: style.css - This is the main file that contains the theme's stylesheet. It includes the theme's name, description, author, and other important information that WordPress uses to identify and display the theme. index.php - This is the main template file that WordPress uses to display the content of your site. It's the file that contains the HTML, CSS, and PHP code that defines the layout and design of your site. However, to create a fully functional WordPress theme, you will need to create additional files such as header.php, footer.php, sidebar.php, functions.php, and more. These files are used to add functionality, customize the design, and create different templates for different pages on your site.