Improved responsiveness on small screens

* better responsiveness and alignment especially on narrower screens
* removed rounded borders for drum pads and main card

drum-machine/public/index.html

@@ -36,5 +36,6 @@
       To begin the development, run `npm start` or `yarn start`.
       To create a production bundle, use `npm run build` or `yarn build`.
     -->
+    <script src="https://cdn.freecodecamp.org/testable-projects-fcc/v1/bundle.js"></script>
   </body>
 </html>