Add background color on key verification screen

// FREEBIE

stylesheets/_conversation.scss

@@ -27,6 +27,8 @@
 }
 
 .key-verification {
+  background: white;
+
   p {
     padding: 1em;
   }

stylesheets/manifest.css

@@ -510,6 +510,8 @@ input.search {
 .conversation, .message-list, .message-detail, .key-verification {
   height: 100%; }
 
+.key-verification {
+  background: white; }
   .key-verification p {
     padding: 1em; }
   .key-verification .key {