One of the goals of the high-contrast themes is to provide clarity for viewers. Shadows on text or other graphics can provide a very nice, subtle 3D effect, but this can be troublesome for some people. When designing styles for high-contrast, it's important to remember to deliberately remove such shadows, as illustrated in the example below.
This button uses a white drop shadow on the black text to provide an elegant embossed look:
But such a shadow would be inappropriate for a high contrast theme. In this example (the yellow-on-black theme in this case), the text shadow is removed completely: