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Check Hello Dolly plugin that comes with WordPress;

// File: wp-content/plugins/hello.php
// We need some CSS to position the paragraph
function dolly_css() {
    // This makes sure that the positioning is also good for right-to-left languages
    $x = is_rtl() ? 'left' : 'right';

    echo "
        <style type='text/css'>
            #dolly {
                float: $x;
                padding-$x: 15px;
                padding-top: 5px;       
                margin: 0;
                font-size: 11px;
            }
        </style>
    ";
}
add_action( 'admin_head', 'dolly_css' );

Like @benoti mentioned use wp_head hook in you don't need the admin side:

add_action('wp_head', 'function_callback' );

BTW, this plugins appends styles and the same works for scripts.