WordPress Theme Files Not Linking

You may want to try /styles.css , or ./styles.css . The "." means "up one directory level ".

However, a better way to do it, would be to link to it like this:

<link rel="stylesheet" href="<?php echo get_stylesheet_directory_uri(); ?>/styles.css" type="text/css" />

This creates a dynamic absolute path to the stylesheet.

As @rambu said below, wp_enqueue_style is the proper way to do this.

function add_my_style() {
    wp_enqueue_style( 'my-styles', get_stylesheet_directory_uri().'/style.css' );
}

add_action( 'wp_enqueue_scripts', 'add_my_style' );