WordPress : Add js and css in your custom theme

In your theme  functions.php add your css and js like:

function cosmowhiz_styles__scripts() {
        // Bootstrap Core CSS
        wp_enqueue_style( 'bootstrap.min', get_template_directory_uri() . '/bootstrap/css/bootstrap.min.css',false,'3.3.7','all');
        
        // Custom Fonts
        wp_enqueue_style( 'font-awesome.min', get_template_directory_uri() . '/font-awesome/css/font-awesome.min.css',false,'4.6.3','all');
        wp_enqueue_style( 'google-fonts-lora', 'http://fonts.googleapis.com/css?family=Lora:400,700,400italic,700italic', false );
        wp_enqueue_style( 'google-fonts-montserrat', 'http://fonts.googleapis.com/css?family=Montserrat:400,700', false );
        // Theme CSS 
        wp_enqueue_style( 'grayscale.min', get_template_directory_uri() . '/css/grayscale.min.css',false,'4.6.3','all');
        // HTML5 Shim and Respond.js IE8 support of HTML5 elements and media queries
        // WARNING: Respond.js doesn't work if you view the page via file:
        wp_enqueue_script( 'html5shiv', 'https://oss.maxcdn.com/libs/html5shiv/3.7.0/html5shiv.js' );
        wp_script_add_data( 'html5shiv', 'conditional', 'lt IE 9' );
        wp_enqueue_script( 'respond.min', 'https://oss.maxcdn.com/libs/respond.js/1.4.2/respond.min.js' );
        wp_script_add_data( 'respond.min', 'conditional', 'lt IE 9' );
        // Bootstrap Core JavaScript
        wp_enqueue_script( 'bootstrap.min.js', get_template_directory_uri() . '/bootstrap/js/bootstrap.min.js', array('jquery') ,'3.3.7');
        // Plugin JavaScript 
        wp_enqueue_script( 'jquery.easing.min', 'http://cdnjs.cloudflare.com/ajax/libs/jquery-easing/1.3/jquery.easing.min.js');
        //Google Maps API Key - Use your own API key to enable the map feature. More information on the Google Maps API can be found at https://developers.google.com/maps/ -->
        wp_enqueue_script( 'maps', 'http://maps.googleapis.com/maps/api/js?key=AIzaSyCRngKslUGJTlibkQ3FkfTxj3Xss1UlZDA&sensor=false');
        //Theme JavaScript
        wp_enqueue_script( 'grayscale', get_template_directory_uri() .'/js/grayscale.js');
    }
    add_action( 'wp_enqueue_scripts', 'cosmowhiz_styles__scripts' );