The border is not explicitly defined within the css, it is there by default when the <fieldset> tag is used. If you would like to get rid of the border you need to set the border to none in the following style:
form.NewFromBlank_Default fieldset.NewFromBlank_Default {
border: none; /*add in this attribute to get rid of the border */
clear:both;
}
If you have a different style applied to your field set you will need to add this attribute to that style.