MDL-42434 mod_chat: styles for chat themes to view help-icon properly

The styles for the bubble and the compact themes have been reviewed
because there weren't enough contrast for the help icon.

mod/chat/gui_ajax/theme/bubble/chat.css

@@ -242,3 +242,7 @@ Dual licensed under MIT and GNU GPLv2 © Nicolas Gallagher
     border-width: 9px 0 9px 21px;
     border-color: transparent #fff;
 }
+
+#button-send + a > i.text-info {
+    color: black !important;
+}

mod/chat/gui_ajax/theme/compact/chat.css

@@ -95,3 +95,7 @@
     background: #e3e8f0;
     height: 100%;
 }
+
+#button-send + a > i.text-info {
+    color: black !important;
+}