How can I customize the border of the check box at the very bottom of the form?

How can I customize the border of the check box at the very bottom of the form? I have tried to see if there's a custom CSS label for just the box itself, and can't seem to locate that. In the designer, there is no option for the specific box I selected to change the border. Changing the color only changes the color of the checkmark that appears.

What changes are you trying to apply to the field? 

This is the code to change the border color of the check box, and you may add other properties not only to change the border color:

.form-checkbox-item:not(#foo) label:before {

    border-style: solid;

    border-color: yourcolorhere;

}

This CSS code will help you to change the color of the text:

.form-checkbox-item:not(#foo) label{

    color: yourcolorhere;

}

If this does not help, please provide us a bit more about what are you trying to achieve, we will be glad to help you.