A WordPress theme is a set of files, created using a predefined structure and features, to act as the presentation layer of the website. In simple terms, the presentation layer should contain the HTML needed to generate the layout and all the data passed by the models. WordPress is built to create content management systems, and hence it doesn't focus on separating the presentation layer from its business logic.
Themes contain template files as a mix of both HTML code and PHP logic. As a developer, you need to have knowledge of both designing layouts and applying logic to work with themes.
The themes in your WordPress site are located in the
wp-content/themes directory, with each theme using its own folder. A theme is identified by the predefined set of comments used in the
style.css file. If this file is not available or the comment is broken, WordPress will not list it as a theme, even though the theme files have been placed in the