[Vala] How to change the widget color

[zeta <kmsiete gmail com>]

Hi there

I try to change the color of the button, scale, and progressbar 
widgets, it seem this can be achived with a StyleContext but I can't 
figure how to use it, the other option I think is using  css but I 
don't find any example on how to do this with vala, If anyone can share 
an example is very welcome.

Regards
Miguel Angel