[cxr]

Zotero and, previously, the UI updates to MonoDevelop (Xamarin Studio) are both proof that in order to get the best UI for all platforms, you should work on making it look decent-to-good (if not perfectly suited) for Mac OS and then just use the same look for Windows and Linux. You won't be able to completely avoid it seeming at least somewhat out of place on the latter two, but it also won't look bad.