How to choose platform for WPF and Winforms

Hi,
I am developing an application Commercial ERP software for my client. The software should be cross-platform (Win, Mac-OS X, Linux) and language is C# to be used. I like the WPF look and Feel and Ribbon Style look and feel. But it seems it does not support cross platform. Which platform should I choose to develop this style of application? I need to take urgent decision, please help with your input and guidance.