diff --git a/theme/index.hbs b/theme/index.hbs
index d09161f5..b08b8364 100644
--- a/theme/index.hbs
+++ b/theme/index.hbs
@@ -173,9 +173,31 @@
                 <script type="text/javascript">
                     document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible');
                     document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible');
+
+                    // Get viewed page store
+                    var viewed_key = 'mdbook-viewed';
+                    var viewed_map = {};
+                    try {
+                        var viewed_storage = localStorage.getItem(viewed_key);
+                        if (viewed_storage) {
+                            viewed_map = JSON.parse(viewed_storage)
+                        }
+                    } catch (e) { }
+
                     Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) {
                         link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1);
-                    });
+                        
+                        // Apply viewed style
+                        if (viewed_map[link.pathname]) {
+                            link.classList.add('md-viewed')
+                        }
+                    }); 
+
+                    // Mark viewed after 30s
+                    setTimeout(function() {
+                        viewed_map[location.pathname] = 1;
+                        localStorage.setItem(viewed_key, JSON.stringify(viewed_map));
+                    }, 30000)
                 </script>
 
                 <div id="content" class="content">
diff --git a/theme/style3.css b/theme/style3.css
index b05d5a73..1450230f 100644
--- a/theme/style3.css
+++ b/theme/style3.css
@@ -113,3 +113,6 @@ code.editable, .ace_scroller {
     margin-top: 8px;
 }
 
+.chapter li .md-viewed {
+    color: #777;
+}