add theme picker

lib/html_body.typ

@@ -1,4 +1,5 @@
 #import "./custom_html.typ" as chtml
+#import "./theme_picker.typ": theme_picker
 
 /// Make the body of the webpage
 #let html_body(
@@ -10,9 +11,43 @@
   logo_alt,
   body
 ) = {
+  ```raw-css
+
+  site-wrapper {
+    display: flex;
+    flex-wrap: wrap;
+    width: 100%;
+    justify-content: center;
+
+    nav {
+      display: flex;
+      theme-picker {
+        margin-left: auto;
+      }
+    }
+  
+    site-container {
+      width: 1050px;
+      padding: 0 20px;
+  
+      header {
+        // display: flex
+        // flex-wrap: wrap;
+        justify-content: space-between;
+        align-items: center;
+        padding: 15px 5px;
+        border-bottom: solid 1px var(--color-border);
+        margin-top: 15px;
+      }
+    }
+  }
+  ```
   html.body({
     chtml.site-wrapper({
       chtml.site-container({
+        html.nav({
+          theme_picker()
+        })
         html.header(style: "display: flex; flex-wrap: wrap-reverse;", {
           html.hgroup({
             header