Make iOS "safe area" respect dark mode (#2053)

src/components/App.tsx

@@ -724,7 +724,7 @@ class App extends React.Component<ExcalidrawProps, AppState> {
       });
     }
 
-    document.body.classList.toggle(
+    document.documentElement.classList.toggle(
       "Appearance_dark",
       this.state.appearance === "dark",
     );

src/css/theme.scss

@@ -37,6 +37,10 @@
   --popup-text-inverted-color: #{$oc-white};
 }
 
+:root.Appearance_dark {
+  background-color: #000;
+}
+
 .Appearance_dark {
   --text-color-primary: #{$oc-gray-4};
   --bg-color-island: #1e1e1e;