Sidebox Heading text color

I'm not sure im looking in the right place but my question is about the text color in the sidebox headings.
For example, the sidebox is called categories, and the default color is white - i changed the sidebox background so the word categories cant be seen - how can i change that to black or a different color, i thought it was in style sheets but i cant seem to find it. I searched the FAQ's but everything i find seems to relate to the links inside the box, I am only concerned with the title ie categories, whats new, information etc.
thanks

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