Styling the Field Label of a Ninja Form

nf-css-field-label.css

/*
Styling all field labels
*/
.field-wrap label {
  /* Your CSS styles */
}

/* 
Styling a field type labels
EXAMPLE: .textbox-wrap label {}
*/
.field-type-name-wrap label {
  /* Your CSS styles */
}

/* 
Styling a specific field label
EXAMPLE: #nf-field-12-wrap label {}
*/
#nf-field-field_ID-wrap label {
  /* Your CSS styles */
}