Update workbook.rb

Add support to fill colors that are picked using theme colors.
![screen shot 2014-10-16 at 4 01 10 pm](https://cloud.githubusercontent.com/assets/103536/4669583/5a59ee8a-556f-11e4-879c-c31834b1b3c5.png)
As of now, fill colors chosen as indicated above are not detected by get_fill_color.