Inconsistency in applying style

Trying to set the background color of some columns we ran into very strange behavior of the grid. In our case we do all the changes AFTER the grid is displayed since we dynamically want to change the style of the cols (upon user preferences) :

*obj.getCellTemplate(1).setStyle("background-color", "green"); does not work...until i do a sort ?!

*Trying to set with CSS is not clear to me ...
On this subject could someone explain the way it works

Thanks
Jonathan