Check Hello Dolly plugin that goes with WordPress;
File: wp-content/plugins/hello.php
79: // We need some CSS to position the paragraph
80: function dolly_css() {
81: // This makes sure that the positioning is also good for right-to-left languages
82: $x = is_rtl() ? 'left' : 'right';
83:
84: echo "
85: <style type='text/css'>
86: #dolly {
87: float: $x;
88: padding-$x: 15px;
89: padding-top: 5px;
90: margin: 0;
91: font-size: 11px;
92: }
93: </style>
94: ";
95: }
96:
97: 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', ...
BTW, this plugins appends styles, the same is for scripts.