_content/tour: add dark theme cursor and selection styles

Add styles for dark theme cursor and selection.
Cursor color set equal to the foreground color.
Focused selection color set to the complement color
of the background color and inactive selection
is the lighter tint of it.

Fixes golang/go#63313

Change-Id: If9702f45262eea330d42ec0fd9717b36da026540
GitHub-Last-Rev: 1545bbaea9
GitHub-Pull-Request: golang/website#253
Reviewed-on: https://go-review.googlesource.com/c/website/+/531856
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Suzy Mueller <suzmue@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Suzy Mueller <suzmue@golang.org>

_content/tour/static/css/app.css

@@ -396,6 +396,13 @@ a#run, a#kill {
 [data-theme='dark'] .cm-s-default .cm-comment {
     color: #fb6;
 }
+
+[data-theme='dark'] .CodeMirror-cursor { border-color: #BABABA; }
+[data-theme='dark'] .CodeMirror-selected { background: #55483a; }
+[data-theme='dark'] .CodeMirror-focused .CodeMirror-selected { background: #433425; }
+[data-theme='dark'] .CodeMirror-line::selection, .CodeMirror-line > span::selection, .CodeMirror-line > span > span::selection { background: #433425; }
+[data-theme='dark'] .CodeMirror-line::-moz-selection, .CodeMirror-line > span::-moz-selection, .CodeMirror-line > span > span::-moz-selection { background: #433425; }
+
 @media (min-width: 601px) {
     #editor-container {
         position: fixed;