Fix syl20bnr theme on OS X

OS X uses a version of `cut` that doesn't support this theme's required `--output-delimiter` flag. To get around this, you can install GNU coreutils from Homebrew, which prefixes its versions of these utils with g (so eg. `gcut`).

I've abstracted the calls to `cut` in the theme in a function that checks whether `gcut` is in the path and uses that instead if so.