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I cloned your form and I found out that there is some existing CSS inside of your Designer/CSS.
You can change the color of submit button there. Inside of your Designer CSS click on your button and you should get code like this.
You need to change Hex values of background (e.g. #F69C06) for some other color of your choice. You can find Hex values of color on this link: http://www.w3schools.com/tags/ref_colorpicker.asp
Let us know if you need further assistance.