fix(web): prevent flashing white background in dark mode on page load/reload (#19934)

* prevent flashing white background in dark mode on page load/reload, tested with Safari and Chrome on macOS

* integrate into existing FOUC-prevention

---------

Co-authored-by: Evan <evan@MacBook-Pro.local>

web/src/app.html

@@ -21,6 +21,11 @@
       html {
         height: 100%;
         width: 100%;
+        background-color: rgb(255, 255, 255);
+      }
+
+      html.dark {
+        background-color: rgb(10, 10, 10);
       }
 
       body,
@@ -29,6 +34,10 @@
         padding: 0;
       }
 
+      body {
+        transition: background-color 0.15s ease;
+      }
+
       @keyframes delayedVisibility {
         to {
           visibility: visible;