Fix issue30 color picker pushbutton not work

The issue #30 is related to pixmap based style.
This style only use on Windows and MacOS, thus only these platform run into these problem

The detail can be found here:
http://qt-project.org/faq/answer/why_does_nothing_happen_when_i_set_the_palette_background_color_on_a_qpushb

To solve this, I use stylesheet property instead and solve this issue.
However this cause a little redundant code since we cannot get color in stylesheet.