A WordPress theme is actually a collection of files in a directory. There are no special or unusual formats, just a few requirements for those files in the theme directory. The only requirements for a directory to be a valid WordPress theme are as follows:
- It should have a style.css file and an index.php file
- The style.css file must have the basic theme information in its initial lines
There are a number of additional files that you'll find in most themes. They are as follows:
- A screenshot.png file that is the little thumbnail that shows what the theme looks like.
- An images directory where all images associated with the theme reside.
- A variety of files that are used for different purposes (for example, functions.php, header.php, footer.php, page.php, single.php, and archive.php). Read the following article to learn about the most commonly used files: https://codex.wordpress.org/Theme_Development#Template_Files.
You don't have to worry about these details now, but knowing them will help you identify what is going on inside the themes that you download. This will also be useful in the following chapters, where we will discuss making our own theme from scratch.
Also, don't worry if you download a theme and its directory structure looks very different from what's described here. Some theme developers decide to go with their own structure to provide some extra features and a more customizable environment. This is mostly the case with various theme frameworks and big premium themes that come with their pre-made child themes.