in js/drill.js [179:186]
add_expand_contract_buttons : function() {
var expand_btn = '<span class="expand show"><i class="fa fa-plus"></i></span>';
var contract_btn = '<span class="contract"><i class="fa fa-minus"></i></span>';
$('li.toctree-l2').filter(function(){
return $(this).next('ul').length > 0;
})
.prepend(expand_btn + contract_btn);
},