Why I don't see upload and preview buttons when using overlay feature?
using an overlay feature have some limitations because this add-on is a template modification only. Upload and preview buttons won't be visible in overlay form and also in static page. If you don't allow users to upload files within conversations there won't be a problem at all. If you disable this feature you'll see both buttons back in static page.
Can I change overlay style?
not through style properties of this add-on. You can change it elsewhere, but it will affect all other overlays.
Can I change the white background in editor in overlay form?
Why I don't redirect to conversation page after creating one?
if you're using overlay function you won't be redirected to conversation page after creation, but will stay on same site, which is useful if you mostly send messages from postbit button. When you start a new conversation from conversation panel (or when clicking on default XenForo Start a Conversation link), you won't be redirected to conversation page too, but you'll get a quick notice in top of page that conversation has been created
if you don't like what you've read above, you can still use default XenForo redirect behaviour by setting the correct option in style properties for this add-on
Does it works with [bd] Widget Framework add-on?
Does is work with Conversation Essentials add-on?
could be, but you need to set Execution order to 9 for conversation_add template modification for Conversation Button under Avatar add-on (debug mode must be enabled).
Does it works with other styles?
it's designed and tested to work on default Xenforo style, so no guarantees to work for 3rd party styles. But here are fixes for styles users report it: