I don't know about lights in the parks, as I was only there during the day, but there are definitely some decorations up. The xmas ball garlands and hanging things are all over New Orleans Square, and there are garlands like that up in parts of DCA, too. I have pictures of those in my most recent trip report.
There are also funky cool spiky ball lights in the trees in DTD, and they are lit up (I left the parks and was walking through DTD as dusk started) once it starts getting dark.