add login route and login form

src/theme/mixins.scss

@@ -1,5 +1,20 @@
+@import "./variables";
+
 @mixin desktop {
     @media (min-width: 800px) {
         @content;
     }
 }
+
+@mixin page-content-wrapper {
+    margin: 10px;
+    padding: 10px;
+    background-color: $color-white;
+    border-radius: 15px;
+    border: $border-large;
+
+    @include desktop {
+        padding: 20px;
+        width: 400px;
+    }
+}