Theory Velour Slim Tank in Green. - size XS (also in ) Theory Velour Slim Tank in Green. - size XS (also in ) 70% viscose 28% nylon 2% elastane. Dry clean only. Velvet fabric. Made in USA. THEO-WS867. I0926502. Theory is a visionary concept for modern intelligent fashion forward people. Founded in New York and made by the finest of fabrics, this makes the working everyday person feel powerful, ready to face their day.