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The easiest way to do this through Form Designer is to go to the CSS tab. Once you are there, you can simply double click on a single field and you will see the change from a rule starting with a dot, starting with a number symbol - #.
this is ID and means that that specific field will have its background color changed - you would need to select the background-color attribute from the list.
I do suggest taking a look at the following guide: Video Course: Form Designer Basics
It will show you several videos, each holding a piece of a puzzle, and once you go through them, I believe that it will seem very easy, but if you happen to have any questions in regards to it, do let us know.