🐛(frontend) fix background color during dnd

When we were dragging an item in the doc tree,
the background color was opaque, making it difficult
to see the underlying content. The cause was
that we were overriding the transparent background
color.

src/frontend/apps/impress/src/features/docs/doc-tree/components/DocSubPageItem.tsx

@@ -110,6 +110,10 @@ export const DocSubPageItem = (props: TreeViewNodeProps<Doc>) => {
             background: var(--c--theme--colors--greyscale-100);
           }
         }
+
+        .row.preview & {
+          background-color: inherit;
+        }
       `}
     >
       <TreeViewItem