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In Drupal 7, a theme function is a PHP function that is used to wrap the output variables with HTML. For adding a theme function in our custom Drupal 7 module, we need to implement hook_theme(). A theme function is prefixed with 'theme_'. However, they can't override existing theme functions. Such functions are invoked using theme() rather than being directly called. Also, they are faster compared to the template (.tpl) file implementations.