Fixed mobile scrolling issues

fixes #12

- added background color to the mobile menu actions
- disabled root scrolling when the mobile menu is opened which fixes the duplicate scrolling issue

assets/js/main.js

@@ -7,8 +7,10 @@
     burger.addEventListener('click', function () {
         if (!navigation.classList.contains('is-open')) {
             navigation.classList.add('is-open');
+            document.documentElement.style.overflowY = 'hidden';
         } else {
             navigation.classList.remove('is-open');
+            document.documentElement.style.overflowY = null;
         }
     });
 })();