feat: change to dark mode theme

based on dracula midnight colors

memory-bank/activeContext.md

@@ -16,6 +16,7 @@
 - Setting up the Vite/TypeScript/Tailwind environment.
 
 ## Active Decisions
+- **Dark Mode Implementation**: Adopted a Dracula Midnight theme (`#21222c` background) as the primary interface style. Added custom Dracula color palette to `tailwind.config.js`.
 - Using **Vite** for the build tool.
 - Using **Vanilla TypeScript** to avoid framework overhead.
 - Using **Vitest** for unit testing calculation logic.