Add CSS for whitespace and tabs

Change-Id: Ibb47077a0f74ec3b72848d0feac9a49db6388b0e

gr-editor/gr-editor.html

diff --git a/gr-editor/gr-editor.html b/gr-editor/gr-editor.html
index 930312b..5ea404e 100644
--- a/gr-editor/gr-editor.html
+++ b/gr-editor/gr-editor.html
@@ -63,6 +63,16 @@
         /* CodeMirror has a default z-index of 4. Set to 0 to avoid collisions with fixed header. */
         z-index: 0;
       }
+      .cm-tab:before {
+        color: #757575;
+        content: "\2192";
+        position: absolute;
+      }
+      .cm-trailingspace {
+        background-color: #ef9a9a;
+        border: 1px solid #f44336;
+        border-radius: 3px;
+      }
     </style>
     <div id="wrapper"></div>
   </template>