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So, first of all, back up your site tree to root:
Click on "Default Styles":
And click Edit:
Notice the file being referenced is already there. The default directory is /farcry/project_name/www/css/. In this case I've changed main.css to all.css. There is a nice textarea displaying the contents of the file, a checkbox for "use CSS in this folder only" and a media type selector.
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