It's basically a whackload of text shadows. If you look at the source, you'll see this inside the SPAN's style code:
HTML:
<span class="smallfont" style="color: white; text-shadow: 2px 0 0 green, -2px 0 0 green, 0 2px 0 green, 0 -2px 0 green, 1px 1px green, -1px -1px 0 green, 1px -1px 0 green, -1px 1px 0 green; cursor: pointer;">
To put that in your own text, use the [span] tag and wrap it around the text of your choice. So it'd look like this:
Code:
[PLAIN][span="color: white; text-shadow: 2px 0 0 green, -2px 0 0 green, 0 2px 0 green, 0 -2px 0 green, 1px 1px green, -1px -1px 0 green, 1px -1px 0 green, -1px 1px 0 green;"]Hello world![/span][/PLAIN]
(I've omitted the cursor: part because generally you won't need it.)
I hope you get what you're looking for with that.