making background transparent on implicit_plot3d

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Plots produced using implicit_plot3d have white background. To get transparent background, I tried using transparent=True in implicit_plot3d and in save, without success:

sage: x, y, z, t = var('x y z t')
sage: name='/Volumes/abcd/Users/efgh/Desktop/ijkl.png'
sage: myplot = implicit_plot3d(z - sqrt(8 - x^2 - y^2),
....:                          [x, -4, 4], [y, -4, 4], [z, -4, 4],
....:                          color='red', axes=True, transparent=True)
sage: myplot.save(filename=name, axes=True, transparent=True, fontsize='12')

The image I obtain has white but not transparent background:

I know implicit_plot3d has an opacity option to set the opacity of the plotted surface, but how can I set the transparency of the background?