You need to escape name of class here.
Change
$('.some_div').append("hey!");
to
$('.some_div').append("hey!");
OR
$('.some_div').append("hey!");
DEMO here.
Demo with style:
$('.some_div').append("hey!");
OR
$('.some_div').append("hey!").find("li:last").css("color","yellow");
Demo here.