Unthemed Windows & Mac OS will be convincing to a lot, Linux will be pretty hard. Then again those who tinker with their computers are unlikely to fall into this trap.
This specific phishing website mimics an ingame website for Counter-Strike Global Offensive, a shooter game with the vast majority of players using Windows. Linux isn't supported at all and while the game technically runs on a Mac most people don't (or play it with Bootcamp).
I would not be surprised if 99% of the audience for this website is using Windows, the vast majority with default themes (and the ones without either won't notice this or think Windows is buggy when a popup shows the default theme).
