frontend: system theme: auto light/dark mode

src/frontend/css/app.css

@@ -447,6 +447,20 @@ button.theme-light {
   background-color: #fff !important;
 }
 
+.theme-swatch {
+  padding: 0;
+  border: none;
+  height: 2.375rem;
+}
+
+.theme-swatch.theme-system {
+  background-image: linear-gradient(135deg, #fff 0, #fff 35%, #0e0e0e 65%, #0e0e0e 100%);
+}
+
+.theme-swatch[aria-pressed="true"] {
+  border: 0.3rem solid #0080d4;
+}
+
 a,
 .btn-link:hover {
   color: #0080d4;
@@ -521,6 +535,39 @@ a,
   border-color: #1a1a1a;
 }
 
+/* theme: system follows the system color scheme */
+
+@media (prefers-color-scheme: dark) {
+  .theme-system,
+  .theme-system .btn-default,
+  .theme-system .dropdown-menu,
+  .theme-system .dropdown-item,
+  .theme-system .form-control,
+  .theme-system .modal-content,
+  .theme-system .toolbar-search {
+    color: #d1d1d1;
+    background-color: #0e0e0e;
+  }
+  .theme-system .content hr,
+  .theme-system .content pre,
+  .theme-system .border-right,
+  .theme-system .border-top,
+  .theme-system .dropdown-divider {
+    border-color: #1a1a1a !important;
+  }
+  .theme-system .selectgroup-label:hover,
+  .theme-system .dropdown-item:hover,
+  .theme-system .toolbar-item:hover,
+  .theme-system .toolbar-search:hover,
+  .theme-system .toolbar-search:focus {
+    background-color: #1a1a1a;
+  }
+  .theme-system .dropdown-menu,
+  .theme-system .modal-content {
+    border-color: #1a1a1a !important;
+  }
+}
+
 /* animation */
 .indicator-enter-active, .indicator-leave-active {
   transition: all .3s;

src/frontend/js/app.ts

@@ -42,6 +42,20 @@ export default {
     })
     this.updateMetaTheme(app.settings.theme_name)
     this.$setLang(app.settings.language)
+
+    // keep the theme-color meta tag in sync when the OS color scheme changes
+    if (window.matchMedia) {
+      this._colorSchemeMql = window.matchMedia('(prefers-color-scheme: dark)')
+      this._colorSchemeHandler = function() {
+        this.updateMetaTheme(this.theme.name)
+      }.bind(this)
+      this._colorSchemeMql.addEventListener('change', this._colorSchemeHandler)
+    }
+  },
+  beforeDestroy: function() {
+    if (this._colorSchemeMql) {
+      this._colorSchemeMql.removeEventListener('change', this._colorSchemeHandler)
+    }
   },
   mounted: function() {
     setupKeybindings(this)
@@ -276,6 +290,10 @@ export default {
   },
   methods: {
     updateMetaTheme: function(theme) {
+      if (theme == 'system') {
+        var dark = window.matchMedia && window.matchMedia('(prefers-color-scheme: dark)').matches
+        theme = dark ? 'night' : 'light'
+      }
       document.querySelector("meta[name='theme-color']").content = this.themeColors[theme]
     },
     refreshStats: function(loopMode) {

src/frontend/js/templates/app.html

@@ -46,14 +46,13 @@
 
                 <header class="dropdown-header" role="heading" aria-level="2">{{ $t('theme') }}</header>
                 <div class="row text-center m-0">
-                    <button class="btn btn-link col-4 px-0 rounded-0"
+                    <button class="btn btn-link theme-swatch col-3 px-0 rounded-0"
                             :class="'theme-'+t"
                             :title="t"
                             :aria-label="t"
                             :aria-pressed="theme.name == t"
                             @click.stop="theme.name = t"
-                            v-for="t in ['light', 'sepia', 'night']">
-                        <v-icon v-if="theme.name == t" name="check" />
+                            v-for="t in ['light', 'sepia', 'night', 'system']">
                     </button>
                 </div>