Check out example codes for "css border left". It will help you in understanding the concepts better.

Code Example 1

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

Code Example 2

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

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

Code Example 3

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

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