No particular theme is required. Songs may be submitted in any common text format, or as graphics in PDF format (particularly handy if you wish to submit sheet music for an original song).
Got a song you want to submit? Email it to batyatoon+contata2014(at)gmail(dot)com, ideally with "Contata 2014 Songbook" somewhere in the subject header. Questions may be sent to the same address or may be asked here as comments.
Please signal-boost this wherever it seems appropriate.