It would help if you told us what fatal error you were getting... ;) 

Although for starters, in your start.php you're registering the wrong event handler. Replace start.php with the following:

<?php

 

function mytheme_init() {
elgg_extend_view('css/elgg', 'Citivend_custom/css');
}

elgg_register_event_handler('init', 'system', 'mytheme_init');
?>