I created a widget called "header-widget" and would like to have "0" margin for this specific widget. I added this to the HTML/CSS Inserts:

div#header-widget { margin: 0 !important;}

But it does not make a difference. Is this the correct way to specify a margin for a specific widget? I don't want to change margins for the other widgets -- only this one.

I love it when I can get it on my own! :D

In case any folks want the answer

I needed to specify a negative margin. That seemed to do the trick.

div#header-widget { margin: -10px !important;}

Good job

Just a general comment: try to avoid !important, unless really necessary.
It makes your code harder to maintain later on.
(But perhaps its necessary in your case.)