Yea, use external .css as you have all forums looking the same. Unless you are planing to make different skins for each forum!
*Jammed, basically .css is a file that tells the browser how our forum should look, the colors, background, size, stuff
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No. CSS (As has been said) Is Cascading Style Sheets. What it does, is keeps a continuous flow with each page of a website you use. It isn't just restricted to forums.
You can keep continuous links, colors, font-faces, ect... with CSS.