Backport PR #2983: Add more border width to CodeMirror cursor (#2994)

notebook/static/notebook/less/codemirror.less

@@ -52,3 +52,20 @@
     .border-radius(0)
 }
 
+.CodeMirror-cursor {
+    border-left: 1.4px solid black;
+}
+
+// When zoomed out 67% and 33% on a screen of 1440 width x 900 height
+@media screen and (min-width: 2138px) and (max-width: 4319px)  {
+    .CodeMirror-cursor {
+        border-left: 2px solid black;
+    }
+}
+
+// When zoomed out less than 33%
+@media screen and (min-width: 4320px)  {
+    .CodeMirror-cursor {
+        border-left: 4px solid black;
+    }
+}