An MkDocs theme controls the visual presentation and layout of the generated site. Themes provide HTML templates, CSS styles, and JavaScript that determine how Markdown content is rendered in the browser. MkDocs ships with two built-in themes (mkdocs and readthedocs), and third-party themes can be installed as Python packages. Material for MkDocs is the most widely used third-party theme, offering search, dark mode, and extensive configuration options.