skin.Graphite.extraStyleSheet in server.properties: how to set multiple css files?

[claudiobosticco]

SmartClient Version: SNAPSHOT_v9.1d_2014-01-21/PowerEdition Deployment (built 2014-01-21)

I'm trying to specify multiple css files with the extraStyleSheet property.
I've tried names separated by comma and by semicolon, without success.
Is it supported to pass a list of files to the extraStyleSheet property?

[claudiobosticco]

any hints?

I've also tried a css file with @import directive but without success.