Raga ho fatto uno script che mi mette a secondo della scelta dell'utente un foglio di style differente , ma mi da due errore

1) non carica nella form i file css
2) mi da un notice

Notice: Undefined index: style in C:\xampp\htdocs\trucchiphp\index.php on line 13

Questo è lo script molto semplice
codice:
<!DOCTYPE html>

<html>
    <head>
        <meta charset="UTF-8">
        <title></title>
        <?php
        $style = "default";
        if($_GET['style'])
            
            $style =$_GET['style'];
        
        $files= array();
        $dh = opendir("styles");
        while($file = @readdir($dh)){
            
            if(preg_match("/[.]css$/", "", $file))
            {
                $file = preg_replace("/[.]css$/", "", $file);
                $files []= $file;
            }
        }
        
        ?>
        <style type="text/css" media="all">@import url(styles/<?php echo ($style);?>.css);</style>
    </head>
    <body>
        <table width="800px">
            <tr>
                <td width='200px' class="menu" valign='top'>
                    
                    <div class="menu-active"><a href="home.php">Home</a></div>
                    <div class="menu-inactive"><a href="faq.php">faq</a></div>
                    <div class="menu-inactive"><a href="contact.php">contatti</a></div>
                </td>
                <td width='600px'  valign='top'>
                    
                    <table class="box">
                        <tr>
                            <td class="box-title">
                                
                                Importanti informazioni
                            </td>
                            
                        </tr>
                        
                        <tr>
                            <td class="box-content">
                                
                                lots of information about important events and stuff
                            </td>
                            
                        </tr>


                    </table>  


                </td>
            </tr>
            
        </table>
        
        <form>
            Style:<select name="style">
                <?php                        
                 foreach ($files as $file){?>
                <option value="<?=($file);?>"
                        <?=($file==$style ? "selected":"");?>>
                <?=($file);?>
                </option>
               <?php } ?> 
                 
                
                
            </select>
            <input type="submit" value="Select">
        </form>
    </body>
</html>