Overriding CSS style

[Yannick POTIN <zaghp outlook fr>]

Hello everyone,

I'm testing the style capabilities of GTK+, and after being amazed by the availability of CSS, I couldn't 
find anything in the documentation about modifying an existing rule.

So, my questions are : is it possible to change the CSS « background-image » of my GTK+ main window using C 
only ? Or do I have to build a new CSS rule and reimport it ? Is the last one even possible without losing 
all other rules ?

Thank you.