This thread is about Bodmer's library. Nothing to do with me. (I may have made the occasional reply)
I have just looked at the Adafruit_GFX class and the Adafruit_ST7735 class.
Neither has a background() method.
Ah-ha. Bodmer has found background() as a method in the TFT library that comes with your IDE.
Personally, I would stick with the Adafruit methods. The TFT library is not widely used (but it is supposed to work with ST7735 displays)
David.