css border left

.leftBorder {
  /*           width style color  */
  border-left: 5px solid black;
}

p {
  border-left: 10px inset green;
}

h1 {
  border-left: 15px solid blue;
}

Container(
  decoration: BoxDecoration(
  	border: Border(
  		left: BorderSide(color: mainColor, width: 3),
  	),
  ),
),