add bgimage style for christmas

packages/app/src/components/Theme/ThemeChristmas.module.scss

@@ -27,7 +27,6 @@ $color-link-wiki-hover: lighten($color-link-wiki, 15%);
 
 .growi:not(.login-page) {
   // add background-image
-  #page-wrapper,
   .page-editor-preview-container {
     background-image: url('/images/themes/christmas/christmas.jpg');
     background-attachment: fixed;
@@ -36,6 +35,18 @@ $color-link-wiki-hover: lighten($color-link-wiki, 15%);
   }
 }
 
+.theme :global {
+  .bg-image-wrapper {
+    position: fixed;
+    width: 100%;
+    height: 100%;
+  }
+
+  .bg-image {
+    object-fit: cover;
+  }
+}
+
 //== Light Mode
 //
 .theme :global {