I think FlightCrew does not check the @import. I think I remember that Valloric said somewhere that FlightCrew does not do a check of the stylesheet at the moment.
If that is the case, than FC would not know of the @import and therefore would correctly say that the stylesheet is not used. The message should say perhaps it is unused in the xhtml.
In this case you could ignore this error, but just because you know the source of the error.
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