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There's a theme applied on the form, I presume that is why the frame doesn't turn transparent, it's being overwritten by the theme style.
You can use this custom CSS code to give a priority styling on the form's background color:
background: transparent !important;
You can paste that custom CSS code under the CSS tab in your Form Designer Tool:
Note that it is transparent, so the color of the background of the form page will be reflected there. Here's mine: https://www.jotformpro.com/form/62295115778969. You can see that it is white because the form's page background is white.
Let us know if you need more help on this.