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');
?>

well I wanted to test a theme so I chose an atomic plug in. It didn't take effect on the site itself. 

i'm wondering if it has something to do with version compatibility. 

Make sure you dont have any other theme below the new theme in the plugin order list. In elgg that last loaded plugin will override the previous views. Also try to clear your cache and run the upgrade.php script.