Post a link so that we can tell you exactly what it is if you haven't yet figured it out.
But it looks like the following is what you need to modify. Add this to your additional.css and just change the border to your desired color and width.
By the way, never edit your default files through ftp, there is no need for that and you will lose any changes you make. Always add your changes or modifications using the additional.css
PHP Code:
.formcontrols {
border-bottom: 1px solid #FFFFFF;
border-left: 1px solid #FFFFFF;
border-right: 1px solid #FFFFFF;
}