You can find my Zenburn for Xamarin theme here: https://gist.github.com/kristopherjohnson/784360f9676b59766678
To use it, download the
.json file, open Xamarin Studio, go to Preferences > Syntax Highlighting, click the Add button, and select the file.
It's not a complete rendition of Zenburn. I've only defined the colors that are used in C# and F# code, and haven't bothered with all of the subtle color distinctions between different varieties of syntax elements.