Transparent Form in firemonkey with a stylebook

I have a application with multiple stylebooks containing styles from delphistyles.com. I want to add a trackbar to allow the user to change the transparency of the form at runtime like you can do in VCL with alphablend.

This post: AlphaBlend in FireMonkey says the following:

To make your form background semitransparent you should set form Transparency property to true and use Fill.Color with alpha value like $AAFFFFFF(with Fill.Kind = bkSolid). in this case form border becomes invisible (at least in Delphi XE2)

But how can I achieve this when my form has a stylebook?

1. Set TForm.Transparency to True
2. Put TPanel on the form with Align = Content
3. Use your TPanel as conteiner for all controls
4. Use TPanel.Opacity for transparency