Rendering background colour setting

[jaehongk]

Hello, first of all, thank you for the great library.

I am trying to change the background colour of the rendered PCB board, which the default setting is (COLORS['fr-4']). I wish to have the background colour as black, where I have tried by manually changing the theme.py

self.background = kwargs.get('background', RenderSettings(COLORS['black']))

but I have failed to see the changes. I have also tried

ctx = GerberCairoContext()
ctx.background_color = (0.0, 0.0, 0.0)

May I know how I may change the background colour settings, please? Thank you.