_content/tour: improve color contrast for dark theme

Updated the color to improve the color contrast when using dark
mode. More or less tried to pick a lighter color of the light
mode color.

Color contrast changes:
- cm.atom: 1.02 -> 6.26
- cm.comment: 2.42 -> 7.6

Fixes golang/go#63229

Change-Id: Ie6257b71d32d8cccf11025e74108e80ae4f9b36d
Reviewed-on: https://go-review.googlesource.com/c/website/+/531215
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>

_content/tour/static/css/app.css

@@ -390,6 +390,12 @@ a#run, a#kill {
 [data-theme='dark'] .cm-s-default .cm-string {
     color: #F585C0;
 }
+[data-theme='dark'] .cm-s-default .cm-atom {
+    color: #7bf;
+}
+[data-theme='dark'] .cm-s-default .cm-comment {
+    color: #fb6;
+}
 @media (min-width: 601px) {
     #editor-container {
         position: fixed;