Show when chat messages are selected.

css/converse.css

@@ -645,6 +645,10 @@ span.spinner.hor_centered {
   border-right: 1px solid #176679;
   border-left: 1px solid #176679;
 }
+#conversejs .chat-message span::selection,
+#conversejs .chat-message::selection {
+  background-color: darkgrey;
+}
 #conversejs .chat-content {
   position: relative;
   padding: 4px;

less/converse.less

@@ -703,6 +703,11 @@ span.spinner.hor_centered {
     border-left:1px solid #176679;
 }
 
+#conversejs .chat-message span::selection,
+#conversejs .chat-message::selection {
+    background-color: darkgrey;
+}
+
 #conversejs .chat-content {
     position: relative;
     padding: 4px;