A theme controls visual presentation. Every theme must have style.css (with the theme header) and index.php.
# Minimal theme structure
my-theme/
|-- style.css # Theme header + CSS
|-- index.php # Main template (fallback)
|-- functions.php # Hooks and features
|-- header.php # Header template
|-- footer.php # Footer template
|-- single.php # Single post
|-- page.php # Static pages
|-- archive.php # Archive pages
|-- 404.php # Not found
|-- screenshot.png # Admin preview
# style.css required header:
/*
Theme Name: My Custom Theme
Version: 1.0.0
Author: Alice
Text Domain: my-theme
*/