In order to embed a form without a background, change the form's background to transparent and then embed it to your site using our regular embed codes. Just set the background to transparent in "Theme" in the topbar of the editor.
From the form editor, navigate to "Theme" from the upper right-hand corner of the editor.
Once in "Theme", open the color picker from either:
In the color picker that appears, set the opacity to 0 (transparent) using either option:
Whether you set the slider or change "A" to 0, it will set the other for you and your background will become transparent.