The Border field configures a border: its color, its width, its radius and, optionally, its style.

Configuration
Go to Modern Fields > Fields groups and add the Border field. Its settings are in the inspector:
| Effect | |
|---|---|
| Enable style | Also lets the user choose the border style: solid, dashed or dotted. |
Display the values
The field stores four values, under the keys color, width, style and radius. Read it with get_field():