How to turn a PHP generated page dark? [duplicate] – Blancer.com Tutorials and projects

I’m serving a dead simple page on the raspberry:

<?php
 
 
require 'Parsedown.php';  # A markdown parser.
 
 
$h = date('H');
if( $h < 5 )
{
    $pic = 'res/cyber_rat.png';
}
elseif( $h < 11 )
{
    $pic = 'res/day_rat.png';
}
elseif( $h < 20 )
{
    $pic = 'res/saint_rat.png';
}
else
{
    $pic = 'res/asm_rat.png';
}
 
$readme = file_get_contents('/opt/rat/README.md');
 
# As a text file
#$readme_html = nl2br(htmlspecialchars($readme));  # Convert line breaks and escape HTML
 
# Or as parsed markdown.
$Parsedown = new Parsedown();
$readme_html = $Parsedown->text($readme);  # Convert Markdown to HTML
 
 
?>
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>rat</title>
    <link rel="icon" type="image/png" href="res/icon.png">
</head>
<body>
    <img src="<?php echo $pic; ?>" alt="Saint Rat" style="max-width: 100%; height: auto;" />
    <pre><span class="inner-pre" style="font-size: 14px"><?php echo $readme_html; ?></span></pre>
 
</body>
</html>

Any idea how to turn this into something a visitor would experience if they were using a “dark theme” browser plugin?