That is a rather generic question that does not have a specific answer. to create a "black and Grey website" there will be many places that you will need to edit.
If you are using Power Store 4.5, there is a design editor in the Admin Panel that will allow to make changes to the design of the web site, other wise, you will need to edit the css.
I suggest using a tool like Fire Bug in Fire Fox to access the CSS for an element directly in the browser to easily find the CSS that effects a specific element, see this article for details on using Fire Bug:
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