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If you look in the directory: /modules/newbb/ you will see a newbb.css file in there also, this is where you make changes to the area you mentioned.
I no longer use newbb 2.0 so have not tried, but rather than keeping track of 2 seperate CSS files, you could just take the info you need from the style.css in newbb folder and add it to your main style.css in your theme directory, I am pretty sure this would work.
Dave