No need to link to the original CSS file, use the tpl API option, not how
$(".fancybox").fancybox({ tpl: { closeBtn: '<a title="Close" class="fancybox-item fancybox-close myClose" href="javascript:;"></a>' } });
Note that I added the myClose class to set my own CSS properties, background image, etc., for example
.myClose { height: 50px; width: 50px; background: #ff0000; }
See JSFIDDLE for an example.
Do the same for prev / next icons. Check API documentation (do a tpl search)
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