You should register and enqueue your styles in your functions.php file.
wp_register_style( 'aileron', get_template_directory_uri() . '/aileron/stylesheet.css', array(), '1.0', 'all' ); wp_enqueue_style( 'aileron' );
Which browser are you using to view your site? Some browser need specific font format:
Are the custom fonts located in the same folder as the stylesheet?
Your font should be called correctly, unless the stylesheet isn't loaded. You can check the source code in a browser to see if the stylesheet is getting loaded.