Warning: Declaration of top_bar_walker::start_el(&$output, $item, $depth, $args) should be compatible with Walker_Nav_Menu::start_el(&$output, $item, $depth = 0, $args = Array, $id = 0)

Nella riga dove segnala l’errore es. functions.php riga 256

sostituisci:

function start_el( $output, $item, $depth, $args ) {

con:

function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {