All tricks below must be placed in CSS field on Plugin's settings page.

Use custom background image

Try to add this CSS for specific tab. Don't forget to change image URL.

#nks-content-1 {
  background-image: url(image.png) !important;
  background-size: cover !important;
}

Change padding for specific tab

#nks-content-1 {
  padding: 30px;
}