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You can actually edit this in the Form Designer. Click the 'Color Scheme' under the Design tab. After that, click the "Simulate Highlight" option. Then, click the options icon and there is a drop down where you can change the background color. Here's a screen capture:
You can also use this custom CSS to make the highlight color transparent:
background-color: transparent !important;
Here's a guide on how to inject custom CSS to the form.
Hope that helps. Thank you.