You might be putting it at the beginning of styles.css, but it is supposed to go in Magento in System, Configuration, (General section) Design, (HTML Head section) Miscellaneous Scripts.
there might be a different way to achieve this but putting this line in the box may work (i got this line from Google Fonts).
<link href='https://fonts.googleapis.com/css?family==Source+Sans+Pro' rel='stylesheet' type='text/css'>
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