Update base.css

Author: seirani

packages/notebook-extension/style/base.css

@@ -280,3 +280,28 @@ body[data-notebook='notebooks']
   overflow: hidden;
   white-space: nowrap;
 }
+
+.jp-skiplink {
+  position: absolute;
+  top: -100em;
+}
+
+.jp-skiplink:focus-within {
+  position: absolute;
+  z-index: 10000;
+  top: 0;
+  left: 46%;
+  margin: 0 auto;
+  padding: 1em;
+  width: 15%;
+  box-shadow: var(--jp-elevation-z4);
+  border-radius: 4px;
+  background: var(--jp-layout-color0);
+  text-align: center;
+}
+
+.jp-skiplink:focus-within a {
+  text-decoration: underline;
+  color: var(--jp-content-link-color);
+}
+