Icons on custom made theme

When I add icons to my menu on my custom made theme, they won’t show. When I activate another theme (twentyfourteen) it’s working very well.

My menu-code in the custom made theme:

<div class="rightcontent">
<?php if ( has_nav_menu( 'right-menu' ) ) : ?>
	<nav id="site-navigation" class="main-navigation" role="navigation">
			<?php wp_nav_menu( array( 'theme_location' => 'right-menu' ) ); ?>
		</nav><!-- #site-navigation -->
	<?php endif; ?>
</div>

Can you help me with this? Thanks!

https://ww.wp.xz.cn/plugins/menu-icons/