I actually want to update our CSS to something better (PRs welcome!)

Initially I picked this theme because it looked really nice in Servo, but we support a lot more CSS now and one of the nicer themes should work well too. I know that the older This Week in Rust theme works in Servo (they recently switched to Bootstrap which mostly works, but has some quirks), and I've wanted to switch to that for a while. Never really sat down to do it, perhaps I should do that next.