Unable to change background color of "Thank You" page to transparent?

Upon checking your form titled "AS Cargo Care - Enquiry", you have added multiple CSS codes and you have set the .thankyou class to white instead of transparent. I presume you wanted it transparent?

To make the "Thank You" page transparent, use this CSS code:

.thankyou, #stage.form-all, #stage.form-all > div {

    background-color : transparent !important;

    border: none !important;

    box-shadow: none !important;

}

I then embedded the form using the iFrame embed code on my website page that has a background color of pink. On the "Thank You" page, you'll see that the background pink is displayed because of the transparency. Here's a screencast showing the effects of the change:

Let us know if that works.