Style codemirror-editor line nums

Also adds a focus call to the editor on attachment.

Change-Id: I15301fb185ede467d6d4b12fbae57061e265e35f
diff --git a/gr-editor/gr-editor.html b/gr-editor/gr-editor.html
index d40ec8f..cc44862 100644
--- a/gr-editor/gr-editor.html
+++ b/gr-editor/gr-editor.html
@@ -65,6 +65,12 @@
         /* CodeMirror has a default z-index of 4. Set to 0 to avoid collisions with fixed header. */
         z-index: 0;
       }
+      .CodeMirror-linenumbers {
+        background-color: #fafafa;
+      }
+      .CodeMirror-linenumber {
+        color: #757575;
+      }
       .cm-tab:before {
         color: #757575;
         content: "\2192";
diff --git a/gr-editor/gr-editor.js b/gr-editor/gr-editor.js
index 4001dab..e608ab3 100644
--- a/gr-editor/gr-editor.js
+++ b/gr-editor/gr-editor.js
@@ -40,7 +40,10 @@
       const params =
           this.getCodeMirrorParams(this.fileType, this.fileContent, this.prefs);
       this.mirror = CodeMirror(this.$.wrapper, params);
-      this.async(() => { this.mirror.refresh(); }, 1);
+      this.async(() => {
+        this.mirror.refresh();
+        this.mirror.focus();
+      }, 1);
       this.addEventListeners();
     },