StoreCreationPlanView: update the preferred color theme using environment modifier.

Author: jaclync

WooCommerce/Classes/Authentication/Store Creation/Plan/StoreCreationPlanView.swift

@@ -154,7 +154,7 @@ struct StoreCreationPlanView: View {
         }
         .background(Color(.withColorStudio(.wooCommercePurple, shade: .shade90)))
         // This screen is using the dark theme for both light and dark modes.
-        .preferredColorScheme(.dark)
+        .environment(\.colorScheme, .dark)
     }
 }