Why are styles not defined?

Question

HTML

Good day. I am using mdbbootstrap.
Registered footer

<div class="footer"> Электронная музыка 2019 <br>
 Copyright &copy;2019
<?php
 require_once $_SERVER['DOCUMENT_ROOT'].'/footer.php'; 
 ?>
 </div>

connection : in style.css file
<link href="css/style.css" rel="stylesheet">

footer {
    align-content: center;
}

Answer 1

Decide whether you are using php or css.