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

Code Example 1

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

Code Example 2

Container(
      child: Text(
          'This is a Container',
          textScaleFactor: 2,
          style: TextStyle(color: Colors.black),
      ),

      decoration: BoxDecoration(
          borderRadius: BorderRadius.circular(10),
          color: Colors.white,
          border: Border(
              left: BorderSide(
                  color: Colors.green,
                  width: 3,
              ),
            ),
          ),
      height: 50,
     ),

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