Massive margin between posts when zoomed out in browser

When you have your browser set to zoom out to 90% or lower you will get a large margin on the viewtopic page between every posts.

This is caused by the CSS calculation in /styles/prosilver/theme/responsive.css:

@media (min-width: 701px) {

 .postbody {

 /** deduct postprofile width (including border width, margin and padding) and postbody margin and padding */

 width: calc(100% - 200px - 1px - 16px);

 margin-right: 16px;

 }

 .postprofile {

 width: 200px;

 }

}

The "border width" is set to 1px, but this is not correct when zoomed out.

Steps to reproduce:

1. Visit https://area51.phpbb.com/phpBB/viewtopic.php?f=81&t=75446
2. Zoom out in browser