To adjust the transparency of your Engage Box go to its Appearance tab and click on the Background Color field for the color picker to reveal itself and help you adjust the transparency along with the color. Evidently, you can drag the transparency marker in the color picker up and down like it is depicted in the screenshot below.
Alternatively, if you want complete transparency, you can also just type the word transparent in the Background Color field like it is depicted in the screenshot below. This way, your box will be 100% transparent.
