Adds the possibility to change the Pygments style

With this change is possible to choose the Syntax Highlight style setting the variable `PYGMETNS_STYLE` to one of the following styles:
- autumn
- borland
- bw
- colorful
- default
- emacs
- friendly
- fruity
- manni
- monokai
- murphy
- native
- pastie
- perldoc
- rrt
- solarizeddark
- solarizedlight
- tango
- trac
- vim
- vs


I removed the statically coded background for highlighted code and adjusted the `style.css` in order to let the pygment `.css` file take care of it. 

To avoid any problems with users the native style (same as before) is adopted as default.

I personally recommend solarizeddark and monokai.