The `rgba` function doesn't quite do the same thing.

I chose not to use `rgba-with-fallback` here because this is not a true fallback. The colors differ depending on whether rgba is supported (in the other usage of `rgba-with-fallback`, the hex background is the color that *would* result if rgba were supported and applied to a white background).

In hindsight, though, I see that `rgba-with-fallback` is a bit strange. And it's only used once. I'll probably just go ahead and remove it.