Fix for issue jupyter#7565 on Click to add a cell button not to be in background

Author: aj202088

packages/notebook-extension/style/base.css

@@ -116,9 +116,14 @@ body[data-notebook='notebooks']
 
 /* Tweak the notebook footer (to add a new cell) */
 body[data-notebook='notebooks'] .jp-Notebook-footer {
-  background: unset;
+  background: var(--jp-layout-color1);
+  margin-left: auto !important;
+  margin-right: auto !important;
+  border-top: 1px solid var(--jp-border-color1);
   width: 100%;
-  margin-left: unset;
+  border: 1px solid var(--jp-border-color1) !important;
+  border-radius: 4px;
+  opacity: 1;
 }
 
 /* Mobile View */