Remove some left over margins from the banner that was just removed (#588)

Author: danny6167

.vuepress/theme/Layout.vue

@@ -1,7 +1,7 @@
 <template>
   <div>
     <div
-        class="theme-container mt-12"
+        class="theme-container"
         :class="pageClasses"
         @touchstart="onTouchStart"
         @touchend="onTouchEnd"

.vuepress/theme/styles/sidebar.css

@@ -6,7 +6,7 @@ $arrow-bg: #000;
     }
 
     @apply .fixed .left-0 .bottom-0 .bg-white .overflow-auto .border-r .border-grey-lighter .py-8;
-    top: calc(3rem + 56px);
+    top: 56px;
     width: 20rem;
     font-size: 15px;