Remember chosen theme in localStorage

This way, the selected theme is kept when changing pages, and for subsequent visits. To prevent flickering, we set the selected theme in a blocking script directly on load. To speed things up further, we now use a `data-theme` attribute instead of classes on the body tag, this way we don't need to wait until the body element is loaded.

blog/sass/css/edition-2/main.scss

@@ -92,7 +92,7 @@ body {
     @include set-colors-light();
 }
 
-body.dark {
+[data-theme="dark"] body {
     @include set-colors-dark();
 }
 
@@ -103,7 +103,7 @@ body.dark {
         @include set-colors-dark();
     }
     /* Override dark mode with light mode styles if the user decides to swap */
-    body.light {
+    [data-theme="light"] body {
         @include set-colors-light();
     }
 }
@@ -966,7 +966,7 @@ img {
 .dark-mode-note {
     display: none;
 }
-body.dark .dark-mode-note {
+[data-theme="dark"] .dark-mode-note {
     display: block;
 }
 @media (prefers-color-scheme: dark) {
@@ -975,7 +975,7 @@ body.dark .dark-mode-note {
         display: block;
     }
     /* Override dark mode with light mode styles if the user decides to swap */
-    body.light .dark-mode-note {
+    [data-theme="light"] .dark-mode-note {
         display: none;
     }
 }
@@ -994,7 +994,7 @@ body.dark .dark-mode-note {
     @include light-switch-light();
 }
 
-body.dark .light-switch {
+[data-theme="dark"] .light-switch {
     @include light-switch-dark();
 }
 
@@ -1003,7 +1003,7 @@ body.dark .light-switch {
         @include light-switch-dark();
     }
 
-    body.light .light-switch {
+    [data-theme="light"] .light-switch {
         @include light-switch-light();
     }
 }

blog/static/js/edition-2/main.js

@@ -1,17 +1,21 @@
-window.onload = function() {
-  var container = document.querySelector('#toc-aside');
+window.onload = function () {
+  let theme = localStorage.getItem("theme");
+  if (theme != null) {
+    set_theme(theme)
+  }
   
+  let container = document.querySelector('#toc-aside');
   if (container != null) {
     resize_toc(container);
     toc_scroll_position(container);
-    window.onscroll = function() { toc_scroll_position(container) };
+    window.onscroll = function () { toc_scroll_position(container) };
   }
 }
 
 function resize_toc(container) {
-  var containerHeight = container.clientHeight;
+  let containerHeight = container.clientHeight;
 
-  var resize = function() {
+  let resize = function () {
     if (containerHeight > document.documentElement.clientHeight - 100) {
       container.classList.add('coarse');
     } else {
@@ -20,8 +24,8 @@ function resize_toc(container) {
   };
   resize();
 
-  var resizeId;
-  window.onresize = function() {
+  let resizeId;
+  window.onresize = function () {
     clearTimeout(resizeId);
     resizeId = setTimeout(resize, 300);
   };
@@ -32,7 +36,6 @@ function toc_scroll_position(container) {
     // skip computation if ToC is not visible
     return;
   }
-  var items = container.querySelectorAll("li")
 
   // remove active class for all items
   for (item of container.querySelectorAll("li")) {
@@ -40,15 +43,15 @@ function toc_scroll_position(container) {
   }
 
   // look for active item
-  var site_offset = document.documentElement.scrollTop;
-  var current_toc_item = null;
+  let site_offset = document.documentElement.scrollTop;
+  let current_toc_item = null;
   for (item of container.querySelectorAll("li")) {
     if (item.offsetParent === null) {
       // skip items that are not visible
       continue;
     }
-    var anchor = item.firstElementChild.getAttribute("href");
-    var heading = document.querySelector(anchor);
+    let anchor = item.firstElementChild.getAttribute("href");
+    let heading = document.querySelector(anchor);
     if (heading.offsetTop <= (site_offset + document.documentElement.clientHeight / 3)) {
       current_toc_item = item;
     } else {
@@ -63,25 +66,20 @@ function toc_scroll_position(container) {
 }
 
 function toggle_lights() {
-  var body = document.querySelector("body");
-  var comment_form = document.querySelector("iframe.giscus-frame");
-  if (body != null) {
-    if (body.classList.contains("dark")) {
-      body.classList.replace("dark", "light");
-      if (comment_form != null) {
-        comment_form.contentWindow.postMessage({
-          giscus: { setConfig: { theme: 'light' } }
-        }, "https://giscus.app")
-      }
+  if (document.documentElement.getAttribute("data-theme") === "dark") {
+    set_theme("light")
   } else {
-      body.classList.remove("light");
-      body.classList.add("dark");
-      if (comment_form != null) {
-        comment_form.contentWindow.postMessage({
-          giscus: { setConfig: { theme: 'dark' } }
-        }, "https://giscus.app")
-      }
-    }
-    console.log(body)
+    set_theme("dark")
   }
 }
+
+function set_theme(theme) {
+  let comment_form = document.querySelector("iframe.giscus-frame");
+    document.documentElement.setAttribute("data-theme", theme);
+    if (comment_form != null) {
+      comment_form.contentWindow.postMessage({
+        giscus: { setConfig: { theme: theme } }
+      }, "https://giscus.app")
+    }
+    localStorage.setItem("theme", theme);
+}

blog/templates/edition-2/base.html

@@ -17,6 +17,13 @@
 
     <link rel="alternate" type="application/rss+xml" title="RSS feed for os.phil-opp.com" href="{{ config.base_url | safe }}/rss.xml" />
 
+    <script>
+        let theme = localStorage.getItem("theme");
+        if (theme != null) {
+            document.documentElement.setAttribute("data-theme", theme);
+        }
+    </script>
+
     <script async src="/js/edition-2/main.js"></script>
 
     <title>{% block title %}{% endblock title %}</title>