How can we set the background color of specific textbox field only?

why can't I make an individual text box background color different?

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.