Where is folder of default materials? I have imported my custom materials and since now I have double of everything I want delete default ones but I can't find folder? It's Sketchup 2017.

One quick question;
Where in Sketchup files is located green default background? (actually green and blue if sky is turned on) I want replace it with something different, with some blurry JPEG image which Sketchup not allow by default? Is that possible?