fix: flash of white while loading page in dark mode

Author: adil192

app/lib/dartdoc/dartdoc_page.dart

@@ -102,6 +102,7 @@ extension DartDocPageRender on DartDocPage {
             name: 'viewport',
             content:
                 'width=device-width, height=device-height, initial-scale=1, user-scalable=no'),
+        d.meta(name: 'color-scheme', content: 'light dark'),
         d.meta(name: 'generator', content: 'made with love by dartdoc'),
         d.meta(name: 'description', content: description),
         d.element('title', text: _pageTitle(options)),

app/lib/frontend/templates/views/shared/layout.dart

@@ -59,6 +59,7 @@ d.Node pageLayoutNode({
               name: 'viewport',
               content: 'width=device-width, initial-scale=1',
             ),
+            d.meta(name: 'color-scheme', content: 'light dark'),
             if (noIndex) d.meta(name: 'robots', content: 'noindex'),
 
             // <!-- Twitter tags -->

pkg/web_css/lib/src/_variables.scss

@@ -156,13 +156,17 @@
 }
 
 .light-theme {
+  color-scheme: light;
+
   .displayed-in-dark-theme {
     display: none !important;
   }
 }
 
 /// Variables that are specific to the dark theme.
 .dark-theme {
+  color-scheme: dark;
+
   @include dash_variables.dark-theme;
 
   --pub-color-darkBlack:       #121317;