为什么我的php文件没有响应链接到它的css文件？

Question

我试图更改此php文件上的错误警报的颜色。通过我的CSS文件似乎没有响应。你能帮我弄清楚原因吗？我想让我的警报变得更蓝，更大。

以下是表格：     

<div id="dv_global_form"> 
    <form action="index.php" method="post">
        <p class="required">Required fields *</p>
        Name:<input type="text" name="name" <?php echo"value='$nameValue'"?>>     <span class="required"> *</span><?php echo"<span class='required'>     $nameErr</span>" ;?></span><br>
        Surname:<input type="text" name="surname"<?php echo"value=$surnameValue"?>><span class="required"> *</span><?php echo"<span class='required'> $surnameErr</span>" ;?></span><br>
        Date of Birth:<select name="dob">
        <?php dob()?>
        </select><span class="required"> *</span><?php echo"<span class='required'> $dobErr</span>" ;?><br>
        Email:<input type="email" name="email" <?php echo"value=$emailValue"?>><span class="required"> *</span><?php echo"<span class='required'> $emailErr</span>" ;?>
        <h3>Gender<span class="required"> * </span></h3>
        <input type="radio" name="gender" value="male">Male<br>
        <input type="radio" name="gender" value="female">Female<br>
        <input type="submit" value="submit">

    </form>
    <?php echo"<p class='required' > $genderErr</p>" ;?>
</div>

这里是包含这些功能的php文件：

 function dob(){
 echo "<option></option>";

for($i=1989;$i<2001;$i++){
    if($i==$_POST['dob']){
        echo "<option selected='selected'>$_POST[dob]</option>";
    }else{
        echo "<option>$i</option>";
    }
}
}

 // Validate fields
 $nameErr= $surnamErr=$emailErr= $dobErr=$sexErr=$genderErr="";
 $nameValue=$surnameValue=$dobValue=$emailValue="";

  if($_SERVER["REQUEST_METHOD"]==POST){

if(empty($_POST['name'])){
    $nameErr="Name is required field";
}else{
        if(!preg_match("/^[a-zA-Z\s]*$/",$_POST['name'])){
            $nameErr="Only letters and white space allowed";
        }else{
            $nameValue=$_POST['name'];
        }
}

if(empty($_POST['surname'])){
    $surnameErr="Surname is required field";
}else{
        if(!preg_match("/^[a-zAZ]*$/",$_POST['surname'])){
            $surnameErr="Only letters and white space allowed";
        }else{
            $surnameValue=$_POST['surname'];
        }
     }
if(empty($_POST['dob'])){
    $dobErr="Date of birth is required field";
}

if(empty($_POST['email'])){
    $emailErr="Email is a required filed";
}else{
        if(!filter_var($_POST['email'],FILTER_VALIDATE_EMAIL)){
           $emailErr="This email format is not valid";
        }
        $emailValue=$_POST['email'];
}

if(empty($_POST['gender'])){
 $genderErr="Gender is a required field";
}else{

      }





 }

这就是css：

       .required{
        color:blue;
        }

        #gender{

        color:blue;
       }