Stylesheets don't match

Content

We are attempting to turn on chat.    I have everything working, but in my customer portal, the style doesn't match the other options we already have out there.     I believe I need to change a CSS somewhere.   In fact, I'm pretty sure it is called "site.css", but for the life of me, I can't find it.   

Can someone give me a gentle push as to where in the folder structure this might be?

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Service Cloud