Yo have not opened the php tag correctly in the beginning of your snippet. try adding at least one space after <?php and right before the first //:
<?php // Load the theme stylesheets
function theme_styles() {
// Load all of the styles that need to appear on all pages
wp_enqueue_style( 'main_css', get_template_directory_uri() . '/style.css' );
}
add_action('wp_enqueue_scripts', 'theme_styles');
?>