The answer is “it depends” as usual 🙂
You can do it with hooks and I would recommand action-hook in this case or you can do it on your own.
In both cases : define your function in a plugin, functions.php could be fine but be sure your function is not critical and can be dropped when you change theme.
If you do not want to use action-hooks be sure to test if function exists before using it in a template :
if( function_exists('Ciusan_NBar') ){
Ciusan_NBar();
}
Regarding the function declaration I would recommand you some coding standards, for example :
if( 'yes' === $options['CNB_Showing'] )
It’s better to put variable to the right in case you omit an equal sign.