I am trying to add a CSS file to the front page. I have seen Adding stylesheets (CSS) and JavaScript (JS) to a Drupal 8 theme; I just need to know the HOOK machine name for the front page.

function THEMENAME_preprocess_HOOK(&$variables) { $variables['#attached']['library'][] = 'CSSPATH'; } 

What name should I use for the preprocess function, in order to make Drupal calls it only for the front page?