alttakı alıntıladığım kısım header.php deki mevcut kod dızını hocam orada base url nasıl kullanılmış görebilirsiniz, ayrıca base url localhost/xxx/index.php şeklinde kullanmanız uygun olur localhostta
?php
require_once 'admin/class/settings.php'; ?>
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title><?php echo $settings['title']; ?></title>
<base href="<?php echo $settings['site_url']; ?>">
<!-- Stylesheets -->
<link href="css/bootstrap.css" rel="stylesheet">
<link href="css/revolution-slider.css" rel="stylesheet">
<link href="css/style.css" rel="stylesheet">
<!--Favicon-->
<link rel="shortcut icon" href="admin/dist/img/settings/<?php echo $settings['favicon']; ?>" type="image/x-icon">
<link rel="icon" hrref="admin/dist/img/settings/<?php echo $settings['favicon']; ?>" type="image/x-icon">
<!-- Responsive -->
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=0">
<link href="css/responsive.css" rel="stylesheet">
</head>
<body>