The high constrast icons only make use of black an white to get the maximum contrast possible.
Even more important is the focus on (over)simplification. It is more important to give the icon a distinct shape, so it's easy to tell two icons apart than it is to picture the object precisely. The motto here is avoid detail! I have had a lot of trouble with this, but the focus here is not to make things pretty. We're closer to creating signs here. The clearer/unique the icon is the better.
It is usually required to distort object proportions to make them more visible for visually impaired people. So for example when drawing a pencil, it will need to be more thick, with the “head” taking more space than in real life (compared to the body of the pencil). Typically a pencil scaled down to the icon size constraining the real proportions would be too thin to be clear (see Figure 1.2). It is very useful to test your icon during the design process, either simply by looking at it from distance, semi-closing your eyes or applying a gaussian blur to the rendered bitmap (see Section , “Testing Your Artwork”).
To make the icons visible on any background, the silhouette of the icon is once again stroked with white. It is mostly invisible on light backgrounds, but makes the icon visible on dark.
The only artictic element of the high contrast set that is not necersarily driven by usability is the rounding of the edges. It doesn't, however, make the icons less accessible as you can see by using the methods described in Section , “Testing Your Artwork”. Usually you can achieve this only by making the path stroke rounded, no shape tweaking is necersary. Some graphic editors even have “round corners” effects giving you more control of the rounding.