ThinBorder
A thin border, which is always 1 logical pixel (ignoring the size passed to it).
Used by Tantalum (the default theme) when the high-contrast is enabled. Menu items, buttons etc. are given a ThinBorder, with a borderSize of 3, therefore they normally have 2 pixels of extra padding, but when they are focused/armed/hovered-over, the ThinBorder is replaced by a PlainBorder, and therefore the border takes the full 3 pixels. If we had only used a PlainBorder, and adjusted the borderSize, then items would resize, This would be confusing/distracting.