diff --git a/pubspec.yaml b/pubspec.yaml
index 30450ec..9532db7 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -5,7 +5,7 @@ homepage: https://labs.phundrak.com/phundrak/org-website-backend
 author: Lucien Cartier-Tilet <lucien@phundrak.com>
 
 environment:
-  sdk: '>=2.7.0 <3.0.0'
+  sdk: '>=2.12.0 <3.0.0'
 
 dependencies:
 
diff --git a/web/dart/navbar.dart b/web/dart/navbar.dart
index 61b92ec..d7eecb3 100644
--- a/web/dart/navbar.dart
+++ b/web/dart/navbar.dart
@@ -35,7 +35,7 @@ final icons = {
 };
 
 // Get the current page’s title
-String getPageTitle() => querySelector('title').text;
+String getPageTitle() => querySelector('title')!.text!;
 
 // Create the home button
 Future<Element> makeHome() async {
@@ -45,7 +45,7 @@ Future<Element> makeHome() async {
         'afterBegin',
         Element.a()
           ..attributes['href'] = '/'
-          ..append(makeIcon(icons['home'])));
+          ..append(makeIcon(icons['home']!)));
 }
 
 // Create a clickable icon
@@ -61,7 +61,7 @@ Future<Element> makePages() async {
     ..append(Element.a()
       ..attributes['tabindex'] = '-1'
       ..attributes['href'] = 'javascript:void(0)'
-      ..append(makeIcon(icons['pages'])))
+      ..append(makeIcon(icons['pages']!)))
     ..classes.addAll(['nav-item', 'has-dropdown'])
     ..append(Element.ul()
       ..attributes['id'] = 'drop-page'
@@ -87,23 +87,23 @@ Future<Element> makeShare() async {
     ..append(Element.a()
       ..attributes['href'] = 'javascript:void(0)'
       ..attributes['tabindex'] = '-1'
-      ..append(makeIcon(icons['share'])))
+      ..append(makeIcon(icons['share']!)))
     ..append(Element.ul()
       ..classes.add('dropdown')
       ..attributes['id'] = 'drop-share'
       ..append(makeShareLink(
-          makeIcon(icons['twitter']),
+          makeIcon(icons['twitter']!),
           'https://twitter.com/share?text=${getPageTitle()}'
           '&url=${window.location.href}'))
-      ..append(makeShareLink(makeIcon(icons['reddit']),
+      ..append(makeShareLink(makeIcon(icons['reddit']!),
           'https://www.reddit.com/submit?title=${getPageTitle()}s&url=${window.location.href}'))
-      ..append(makeShareLink(makeIcon(icons['email']),
-          'mailto:?subject=${getPageTitle}&body=${window.location.href}'))
+      ..append(makeShareLink(makeIcon(icons['email']!),
+          'mailto:?subject=$getPageTitle&body=${window.location.href}'))
       ..append(makeShareLink(
-          makeIcon(icons['linkedin']),
+          makeIcon(icons['linkedin']!),
           'https://www.linkedin.com/shareArticle?mini=true&url=${window.location.href}'
           '&title=${getPageTitle()}'))
-      ..append(makeShareLink(makeIcon(icons['facebook']),
+      ..append(makeShareLink(makeIcon(icons['facebook']!),
           'https://www.facebook.com/sharer/sharer.php?u=${window.location.href}')));
 }
 
@@ -125,7 +125,7 @@ Future<Element> makeThemeChanger() async {
       ..attributes['tabindex'] = '-1'
       ..append(Element.span()
         ..style.verticalAlign = 'top'
-        ..append(makeIcon(icons['theme']))))
+        ..append(makeIcon(icons['theme']!))))
     ..append(Element.ul()
       ..classes.add('dropdown')
       ..attributes['id'] = 'theme-dropdown'
@@ -144,7 +144,7 @@ Future<Element> makeToc() async {
     ..append(Element.a()
       ..attributes['tabindex'] = '-1'
       ..attributes['href'] = 'javascript:void(0)'
-      ..append(makeIcon(icons['toc'])));
+      ..append(makeIcon(icons['toc']!)));
 }
 
 // Add a navbar atop of the HTML body, containing:
diff --git a/web/dart/parse_sitemap.dart b/web/dart/parse_sitemap.dart
index 92d38ff..3cac6e0 100644
--- a/web/dart/parse_sitemap.dart
+++ b/web/dart/parse_sitemap.dart
@@ -15,29 +15,29 @@ Future<String> fetchRemoteSitemap() async {
 
 // Parse the list of elements and detect pages from this list
 Map<String, String> detectPages(List<Element> t_sitemap,
-    [String t_prefix, Map<String, String> t_links]) {
+    [String? t_prefix, Map<String, String>? t_links]) {
   t_links ??= <String, String>{};
 
   // parse each element in sitemap
   for (var elem in t_sitemap) {
     // if it’s a link
-    if (elem.innerHtml.startsWith('<a')) {
-      elem = elem.firstChild;
-      final url = elem.attributes['href'];
-      final text = elem.firstChild.text;
+    if (elem.innerHtml!.startsWith('<a')) {
+      elem = elem.children[0];
+      final url = elem.attributes['href']!;
+      final text = elem.children[0].text;
       if (excluded_keywords.contains(text) ||
           excluded_keywords.contains(url.substring(0, url.length - 5))) {
         continue;
       }
-      t_links['/$url'] = (t_prefix == null) ? text : '$text ($t_prefix)';
+      t_links!['/$url'] = ((t_prefix == null) ? text : '$text ($t_prefix)')!;
     } else {
       final prefix = (t_prefix == null)
-          ? elem.firstChild.text.replaceAll('\n', '')
-          : '$t_prefix / ${elem.firstChild.text}'.replaceAll('\n', '');
+          ? elem.firstChild!.text!.replaceAll('\n', '')
+          : '$t_prefix / ${elem.firstChild!.text}'.replaceAll('\n', '');
       t_links = detectPages(elem.children[0].children, prefix, t_links);
     }
   }
-  return t_links;
+  return t_links!;
 }
 
 // This function returns a Map which contains all links to languages detected
@@ -45,13 +45,13 @@ Map<String, String> detectPages(List<Element> t_sitemap,
 Future<Map<String, String>> parseSitemap() async {
   final content = await fetchRemoteSitemap();
   final sitemap = Element.ul()..innerHtml = content;
-  final sitemapNodes = sitemap.querySelector('ul').children;
+  final sitemapNodes = sitemap.querySelector('ul')!.children;
   return detectPages(sitemapNodes);
 }
 
 Future<void> createSitemap() async {
   final sitemap = await parseSitemap();
-  final pages = querySelector('#drop-page');
+  final pages = querySelector('#drop-page')!;
   sitemap.forEach((url, name) {
     final link = Element.li()
       ..classes.add('dropdown-item')
diff --git a/web/dart/reorganize_html.dart b/web/dart/reorganize_html.dart
index 215455c..d195649 100644
--- a/web/dart/reorganize_html.dart
+++ b/web/dart/reorganize_html.dart
@@ -13,7 +13,7 @@ Future<Element> makeHeader() async {
       ..attributes['alt'] = 'Logo'
       ..attributes['height'] = '150px'
       ..attributes['width'] = '150px')
-    ..append(querySelector('h1'));
+    ..append(querySelector('h1')!);
   final subt = header.querySelector('.subtitle');
   if (subt != null) {
     header.append(subt);
@@ -32,8 +32,8 @@ Future<void> wrapTables() async {
 // All images that are not nested inside a link will be linkified to themselves.
 Future<void> linkifyImg() async {
   querySelectorAll('img').forEach((img) {
-    if (img.parent.tagName == 'P') {
-      final link = Element.a()..attributes['href'] = img.attributes['src'];
+    if (img.parent!.tagName == 'P') {
+      final link = Element.a()..attributes['href'] = img.attributes['src']!;
       img.insertAdjacentElement('beforeBegin', link);
       link.append(img);
     }
@@ -61,10 +61,10 @@ Future<void> reorganizeHtml() async {
   await linkifyImg();
 
   // Add navbar to page
-  querySelector('body').insertAdjacentElement('afterBegin', navbar);
+  querySelector('body')!.insertAdjacentElement('afterBegin', navbar);
 
   // Add headet to page
-  querySelector('#content').insertAdjacentElement('beforeBegin', header);
+  querySelector('#content')!.insertAdjacentElement('beforeBegin', header);
 
   // Add correct class to TOC
   final toc = (querySelector('#table-of-contents') ??
@@ -72,7 +72,7 @@ Future<void> reorganizeHtml() async {
         ..attributes['id'] = 'table-of-contents'
         ..innerText = 'Table of Contents Unavailable'))
     ..classes.add('dropdown');
-  navbar.querySelector('#toc-drop').append(toc);
+  navbar.querySelector('#toc-drop')!.append(toc);
 
   // Remove all <br> tags from HTML
   querySelectorAll('br').forEach((br) => br.remove());
diff --git a/web/dart/theme.dart b/web/dart/theme.dart
index 0d2f6ea..20e3f76 100644
--- a/web/dart/theme.dart
+++ b/web/dart/theme.dart
@@ -2,22 +2,19 @@ import 'dart:html' show window, querySelector;
 
 final localStorage = window.localStorage;
 
-Future<void> setTheme([String theme]) async {
-  final currentTheme = theme ?? localStorage['theme'] ??
-  () {
-    final devicePrefersDark =
-    window.matchMedia('(prefers-color-scheme: dark)').matches;
-    return devicePrefersDark ? 'nord' : 'light';
-  }();
-  localStorage['theme'] = currentTheme;
-  querySelector('body')
+Future<void> setTheme([String? theme]) async {
+  theme ??= localStorage['theem'] ??
+      (window.matchMedia('(prefers-color-scheme: dark)').matches
+          ? 'dark'
+          : 'light');
+  localStorage['theme'] = theme;
+  querySelector('body')!
     ..classes.clear()
-    ..classes.add(currentTheme);
+    ..classes.add(theme);
 }
 
 void enableThemeChanger() {
   final themes = <String>['light', 'dark', 'nord', 'black'];
   themes.forEach((theme) =>
-    querySelector('#${theme}Btn').onClick.listen((_) => setTheme(theme))
-  );
+      querySelector('#${theme}Btn')!.onClick.listen((_) => setTheme(theme)));
 }