Background colors with default foreground color.

Hey! Nice project. Just a little improvement (at least for my preferences). If you dislike it, feel free to remove this pull request.

Issue:
The current method of setting the background colors change the
foreground color to white, which looks really bad in dark terminals,
specially the yellow.

Solution:
Setting the last 6 colors as the inversion of the the first 6 colors solves the problem.