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Just to be clear, when you say that you're unable to change the background color, when the Background tool on the Form Styles tab of the Preferences (indicated below) is used, doesn't the background color change?
If so, please do the following:
1. Click the Designer button:
2. In the Color Scheme section of the Design tab, use the Background and Frame selectors to choose the desired colors
3. Once satisfied with your selections, save the jotform
Should you need further help with this, please let us know.