Revert "move css to default"

This reverts commit 6c646bb69d.

_includes/header.html

@@ -1,5 +1,23 @@
 <header class="site-header">
+    <style>
+        header {
+            position: fixed;
+            top: 0;
+            width: 100%;
+            z-index: 1000; /* Ensure it stays above other content */
+            background-color: #fff; /* or any background you prefer */
+            box-shadow: 0 2px 5px rgba(0, 0, 0, 0.1); /* Optional: adds a shadow to make it stand out */
+            }
 
+        body {
+        padding-top: 60px; /* Adjust according to your header's height */
+        }
+
+        body.dark-mode header {
+        background-color: #333;
+        color: white;
+        }
+    </style>
 
     <div class="wrapper">
       {%- assign default_paths = site.pages | map: "path" -%}