The right way to include css style file in wordpress theme or plugin, simple tutorial to include css style file in wordpress, and add css code in header or footer.

How To Include CSS File In WordPress

Using wp_enqueue_style() function and wp_enqueue_scripts action, we will include css style file in wordpress theme or plugin.

Include CSS In WordPress Theme

To include css in wordpress theme, we will use get_template_directory_uri() function to get style file link:

function css_style_to_my_theme(){
    wp_enqueue_style( 'theme-style-bla-bla', get_template_directory_uri() . '/css/theme-style.css', false, null );
}
add_action('wp_enqueue_scripts', 'css_style_to_my_theme');

Include CSS In WordPress Plugin

To include css in wordpress plugin, we will use plugins_url() function to get style file link:

function css_style_to_my_plugin(){
    wp_enqueue_style( 'plugin-style-bla-bla', plugins_url( '/css/plugin-style.css', __FILE__ ), false, null );
}
add_action('wp_enqueue_scripts', 'css_style_to_my_plugin');

Include CSS In Posts And Pages Only

You can include some css style file in single posts or pages, for example:

function css_style_to_my_posts_and_pages_only(){
    if( is_single() or is_page() ){
        wp_enqueue_style( 'my-post-style', get_template_directory_uri() . '/folder-name/post-style.css', false, null );
    }
}
add_action('wp_enqueue_scripts', 'css_style_to_my_posts_and_pages_only');

You can read more about is_single() function or is_page() function.

Wrong Way

Do not include css style file like this:

<head>
    <link rel="stylesheet" href="http://my-website.com/blog/style.css" type="text/css" media="all">
</head>

But you can add CSS code in wp_head or wp_footer action, css code only! not css file! for example in wp_head action:

function css_code_in_header(){
    ?>
        <style type="text/css">
            body{
                background-color: #000000;
            }
        </style>
    <?php
}
add_action('wp_head', 'css_code_in_header');

Now your CSS code will be in <head> html tag.

For example in wp_footer action:

function css_code_in_footer(){
    ?>
        <style type="text/css">
            body{
                background-color: #000000;
            }
        </style>
    <?php
}
add_action('wp_footer', 'css_code_in_footer');

Now your CSS code will be in Footer.

You can read tutorial about include jquery and javascript files.