Implementing a custom built menu from bootstrap 3

Hi.

I’m having trouble setting up my menu in a theme I’ve just built. I have read numerous articles, all telling me to do it in a slightly different way, but to no avail. Here is what I have done thus far:

I’ve referenced the menu in the functions.php file like this:

function register_my_menu() {
  register_nav_menu('sideMenu',__( 'Main Menu' ));
}
add_action( 'init', 'register_my_menu' );

I’ve referenced it in the pages that I need it to show like this:
<?php wp_nav_menu( array( 'theme_location' => 'sideMenu' ) ); ?>

I have my css referenced for the site which includes the menu styling in style.css.
However, when I come to create the menu in WP, it is not picking up any of the styling.

You can see the site here:
http://www.kaitlin-howard.co.uk/wordpress/

How can I get it to pick up the CSS styling?

Thanks for any advice.