Add a rest route to WordPress

gistfile1.txt

//Jack Johannson at https://wordpress.stackexchange.com/questions/306609/how-can-i-fetch-css-from-json-wp-rest-api-response

// syntax for use: http://example.com/wp-json/alexi/v1/output_css_uri

// Register a new rest route and create callback to execute
add_action( 'rest_api_init', 'my_rest_routes' );
function my_rest_routes() {

    register_rest_route(
        'alexi/v1',
        '/output_css_uri/',
        [
            'methods'  => 'GET',
            'callback' => 'output_css_callback',
        ]
    );
}

// Callback function to output the css URI
function output_css_callback() {
    return get_stylesheet_uri();
}