Add common stylesheets

src/styles/common.css

@@ -0,0 +1,8 @@
+body {
+    margin: 0;
+    font-size: 16px;
+}
+
+#app {
+    min-height: 100vh;
+}

src/styles/main.css

@@ -1,10 +1,3 @@
-body {
-    margin: 0;
-    min-height: 100vh;
-    background-color: #ffffff;
-    font-size: 16px;
-}
-
 form {
     display: flex;
     justify-content: center;