Get rid of gradients in the style guide.

As discussed in some mail around one year ago, I find that the gradients of the style guide, although nice, make it harder to read than http://google-styleguide.googlecode.com/svn/trunk/cppguide.xml for instance. As discussed in the same mail, I also never got the website to compile on my Linux machine but @sethladd told me to submit the changes without testing since they were simple.

So there it is, one year later :) I used the colors of http://google-styleguide.googlecode.com/svn/trunk/cppguide.xml but I can't guarantee it looks good since I couldn't test it on my machine.