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/options.html b/src/options.html
index 6daa9ca..8bb4fb2 100644
--- a/src/options.html
+++ b/src/options.html
@@ -20,6 +20,7 @@
         <input type="checkbox" id="loaddrafts"> <label for="loaddrafts" data-i18n="loaddrafts"></label> <span class="experimental-label" data-i18n="experimental_label"></span><br>
         <input type="checkbox" id="increasecontrast"> <label for="increasecontrast" data-i18n="increasecontrast"></label><br>
         <input type="checkbox" id="stickysidebarheaders"> <label for="stickysidebarheaders" data-i18n="stickysidebarheaders"></label><br>
+        <input type="checkbox" id="ccdarktheme"> <label for="ccdarktheme" data-i18n="ccdarktheme"></label> <span class="experimental-label" data-i18n="experimental_label"></span><br>
       </p>
       <h4 data-i18n="profileindicator_header"></h4>
       <p>