I'm not really familiar with php and would like to ask you to help me.
I want to insert
<?php include 'styles.css;?>
<?php require 'styles.css;?>
include is for parsing a PHP file, not copying content.
Just read the file and
echo the contents.
<?= file_get_contents('styles.css') ?>
<?= is a shorthand for
<?php echo. See also: HTML output from web server
Note that the include path in PHP environment may be different from that in the HTML, as understood by the browser. Better use
__DIR__ . '/styles.css' to prevent problems.