pymgym wrote: I can handle css with "normal" fonts, but I don't know how to do with google fonts.
I'm not sure what your problem is with Google fonts. Once such font is loaded on the site, it can be applied with the font-family property just like any standard web font. You actually see how it's done in Firebug, you're having the complete property on your screen, and more so, you actually highlighted it in the screenshot...!!! It can't be more obvious, or?
So, choose the selector, in this case "label", and apply the property you already found:
font-family: Lato,sans-serif;
Since you're "not a newbie", you know you can even test it in Firebug. And you know, that if it doesn't apply, you'll need to add !important and/or classes or ID to give it priority. And you know that you'll need to turn file compression and caches off during the modification, if you have any of it on.