[jQuery] Does addClass overwrite property

[joomlafreak]

I have a very basic question. If I use addClass, will it overwrite a
css property.
Say for example I use

$(".div1").click(funtion(){
$(this).addClass("div_active")
});

The div1 had css property background-color set to white and in the
class div_active background property is black.

Will the addClass make the background of div1 black.
I could not do it and it seems the only property that changes is the
one not defined previously in .div1.
Is there a way to achieve it.

Thanks