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Upon checking the forms you have recently edited I found that all of them have a white background, but this one http://www.jotform.us/form/61934766004154.
It currently has a box shadow effect:
To remove it you need to inject this CSS code:
box-shadow: none !important;
This guide will help you to inject the code o your form: How-to-Inject-Custom-CSS-Codes
However, you can provide us the link to the form you're currently editing, we will be glad to check it and find why the code is not working.
We will wait for your response.