[styles] github.css: fix <code> background color.

- reference
    - #f8f8ff
        - http://www.weekendphp.com/relColors/f8f8ff
    - #f8f8f8
        - http://www.weekendphp.com/relColors/f8f8f8
Merge pull request #350 from kaishuu0123/master

[styles] github.css: fix <code> background color.