Border style of the popup:

• "single" Single line border
• "double" Double line border
• "rounded" Rounded border
• [topleft, top, topright, right, botright, bottom, botleft, left] array for Custom border style

Custom border style:

Each character in the list is used for the corresponding position.

• "topleft" top left corner
• "top" top side
• "topright" top right corner
• "right" right side
• "botright" bottom right corner
• "bottom" bottom side
• "botleft" bottom left corner
• "left" left side An empty string can be used to turn off a specific border, for instance, ["", "", "", ">", "", "", "", "<" ] will only make vertical borders but not horizontal ones.