Fix misaligned top border on ListItem after element

[mbrgm]

Due to the nature of the box model, the specified border on the after element does not line up with the rest of the ListItem's top border. I used the flex model's justifyContent property to center the after content vertically and turned the remaining margin into a padding property.

[mbrgm]

This is an example from the kitchen sink app in Chrome. You can see the top border of the after element not properly lining up.

[natew]

Thanks a lot for this. I had changed this a while back and noticed the bug but never got around to fixing it.