fix(dark): classname cannot be applied to itself (#5120)

Author: ovflowd

styles/layout/_dark-theme.scss

@@ -1,15 +1,15 @@
+body.dark {
+  background-color: $dark-black;
+  color: $white;
+}
+
 .dark {
   color-scheme: dark;
 
   .dark-theme-switcher {
     background-image: url('/static/images/light-mode.svg');
   }
 
-  body {
-    background-color: $dark-black;
-  }
-
-  body,
   .blogpost-meta {
     color: $white;
   }