how to set a local theme?

Strange,

according to the docs, I should be able to define a (local) theme using

  g.setTheme({ bg:'#00FF00' });
  g.clear(false); // to test the new theme

but that does not seem to be respected. Indeed, none of the alternatives I tried had any effect:

  g.setTheme({ bg:'#0F0' });
//  g.setTheme({ bg:'#00FF00' });
//  g.setTheme({ bg:0x00FF00 });
//  g.setTheme({ bg:g.toColor(0,1,0) });
  g.clear(false);

What am I doing wrong?