Using the Colors add-on you can remove the header background color (making it transparent).
If you want a semi transparent header (opacity), you’ll have to create a 4×4 pixel image in Photoshop (or something similar), and give it your desired opacity. Save it as a PNG, and upload it as the header background image (using the Backgrounds add-on).
Be sure to remove the header background color either way.
Hope this helps 🙂