Add CC dark theme option

This commit adds an option to enable a dark theme in the Community
Console. It can be enabled manually with a switch which is injected in
the CC or automatically by syncing it with the OS dark mode setting.

Change-Id: I7506e80a409c5b0190d942c9de88354b6c0cce10
diff --git a/src/common/content_scripts.js b/src/common/content_scripts.js
index 7186037..1bc9a7d 100644
--- a/src/common/content_scripts.js
+++ b/src/common/content_scripts.js
@@ -1,7 +1,8 @@
-function injectStylesheet(stylesheetName) {
+function injectStylesheet(stylesheetName, attributes = {}) {
   var link = document.createElement('link');
   link.setAttribute('rel', 'stylesheet');
   link.setAttribute('href', stylesheetName);
+  if ('media' in attributes) link.setAttribute('media', attributes['media']);
   document.head.appendChild(link);
 }