Changes or sets the theme.
original = viz.getTheme()
blackTheme = viz.Theme()
blackTheme.borderColor = (0.1,0.1,0.1,1)
blackTheme.backColor = (0.4,0.4,0.4,1)
blackTheme.lightBackColor = (0.6,0.6,0.6,1)
blackTheme.darkBackColor = (0.2,0.2,0.2,1)
blackTheme.highBackColor = (0.2,0.2,0.2,1)
object0 = viz.addProgressBar('Changed to black.')
object0.setPosition([.5,.3,0])
viz.setTheme(blackTheme) #Changes all current nodes to this theme
object = viz.addProgressBar('Changed to original')
object.setPosition([.5,.5,0])
viz.setTheme(original,True,object) #Changes the theme for object
object2 = viz.addProgressBar('Created black.')
object2.setPosition([.5,.6,0])