Preprocess theme variables for a specific theme hook.
This hook allows modules to preprocess theme variables for a specific theme hook. It should only be used if a module needs to override or add to the theme preprocessing for a theme hook it didn't define.
For more detailed information, see theme().
$variables: The variables array (modify in place).
Note: this list is generated by pattern matching, so it may include some functions that are not actually implementations of this hook.
function hook_preprocess_HOOK(&$variables) {
// This example is from rdf_preprocess_image(). It adds an RDF attribute
// to the image hook's variables.
$variables['attributes']['typeof'] = array(
'foaf:Image',
);
}