I don't exactly know, but I had to make it in some application... The two I have installed that it could be are called "Simple Text" and "Minimalistic Text" or something like that. I think it's the Simple Text one.
edit the config file by right clicking and edit skin... should be params to modifiy that value. if not edit the main rainmeter config and let me know how that goes.
Found the most awesome wallpaper I've seen in a long time
(clicky for larger view!)
EDIT: And this is the wallpaper http://wallbase.cc/wallpaper/934075
Finally learned about rainmeter, aint played with it much any, back is killing me, but I love how simple I can get my desktop, and still have info displayed....