Add dark mode styling for Contents section in post pages

Fixed Contents section links and parent navigation being hard to read
in dark mode by adding proper dark mode styles for headers, links,
borders, and hover states.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

tuftecms/templates/post.html

@@ -202,6 +202,34 @@
         background-color: #dcfce7 !important;
     }
     
+    body.light-mode .directory-listing {
+        border-top-color: #eee !important;
+    }
+    
+    body.light-mode .directory-listing h2 {
+        color: #111 !important;
+    }
+    
+    body.light-mode .items-list a {
+        color: #333 !important;
+    }
+    
+    body.light-mode .items-list a:hover {
+        color: #000 !important;
+    }
+    
+    body.light-mode .parent-navigation {
+        border-top-color: #eee !important;
+    }
+    
+    body.light-mode .parent-link {
+        color: #666 !important;
+    }
+    
+    body.light-mode .parent-link:hover {
+        color: #333 !important;
+    }
+    
     body.dark-mode article h1 {
         color: #e5e5e5;
     }
@@ -237,6 +265,34 @@
         background-color: #1a3d1a;
     }
     
+    body.dark-mode .directory-listing {
+        border-top-color: #2a2a2a;
+    }
+    
+    body.dark-mode .directory-listing h2 {
+        color: #e5e5e5;
+    }
+    
+    body.dark-mode .items-list a {
+        color: #ccc;
+    }
+    
+    body.dark-mode .items-list a:hover {
+        color: #fff;
+    }
+    
+    body.dark-mode .parent-navigation {
+        border-top-color: #2a2a2a;
+    }
+    
+    body.dark-mode .parent-link {
+        color: #999;
+    }
+    
+    body.dark-mode .parent-link:hover {
+        color: #ccc;
+    }
+    
     @media (prefers-color-scheme: dark) {
         article h1 {
             color: #e5e5e5;