Add warning message when using old thread design

In threads which are using nested replies, a warning will be shown if
the user has enabled the old thread design to notify them that they
won't work properly.

Fixed: twpowertools:139
Change-Id: I393ac187fef22a4c5f062bf99d84c373011d514b
diff --git a/src/static/css/ccdarktheme.css b/src/static/css/ccdarktheme.css
index be6b4d9..ddc4b3f 100644
--- a/src/static/css/ccdarktheme.css
+++ b/src/static/css/ccdarktheme.css
@@ -2289,3 +2289,7 @@
 .TWPT-log-entry.TWPT-log-entry--error {
   color: #ff8A80!important;
 }
+
+.TWPT-warning {
+  background-color: #670505!important;
+}