You can add solid borders to an element using following classes. You're able to change the border color using border color utility classes.

The classes follows b{side}-{size} format, where {size} is one of 0, 1, 2, 3, 4, or 5 values. Also, {side} is one of:

• t: Border top
• b: Border bottom
• l: Border left
• r: Border right
• x: Border left and right
• y: Border top and bottom
• null: All sides