Antonelli is a fluid width subtheme for Bartik, the default core theme for Drupal 7. By default Bartik is fixed width - Antonelli changes the layout to fluid (percentage based) with fixed width sidebars.

Antonelli does not change any styles in Bartik - it only changes the layout from fixed to fluid.

A fluid with option was proposed during the development of Bartik, however the request for this option came too late in the Drupal 7 cycle. Due to the lateness of the proposal it was decided to place a fluid width subtheme in contrib until Drupal 8 when we can again discuss the inclusion of a fluid width option. The original issue is here: #846542: Bartik fluid width subtheme

The name is in reference to Jean Bartiks work colleague Kathleen Antonelli.