Adding CSS to WordPress Theme Via functions.php File

functions.php

<?php
// Load the theme stylesheets
function active_theme_styles()  
{
	// Load all of the styles that need to appear on all pages
	wp_enqueue_style( 'custom', get_stylesheet_directory_uri() . '/css/custom.css' );

	// Conditionally load some thing in the home page 
	wp_register_style( 'slideshow', get_stylesheet_directory_uri() . '/css/slideshow.css' );

	if(is_page('home')) {
		wp_enqueue_style('slideshow');
	}

}
add_action('wp_enqueue_scripts', 'active_theme_styles');