The Word object model does not support setting the font color of a style to a theme color. The code below will generate an error if you try it.

The negative RGB value isn't a constant but can be determined using the following code, where a theme color has already been applied to the style

If you work through each of the colors you can create your own constants. Watch out for tints and shades though!

If you're feeling brave, there is an article that explains theme colors in depth: Word Articles: Colours in 2007