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It's because your background of the form has colour. I'm assuming that you want it to be transparent.
You can do that by following:
1. Edit your form and click Design:
2. There you can see Page Background color, just move this slider all the way to the left:
And that should make your form transparent so the color around it will be the color of the webpage.
Hope it helps.
Let us know if you have further questions.