Allow to not export a diagram background color

[fmteau]

• I agree to follow the Code of Conduct that this project adheres to.
• I have searched the issue tracker for a feature request that matches the one I want to file, without success.

Is your feature request related to a problem? Please describe.
In the past, when you were exporting a drawing as a .png image for example, the background color was not exported (which was perfect in my case).
This was changed recently (probably because of issue #1583) i.e. the background color in the diagram is now also exported in the picture (v24.7.17 for example now behaves like this).

Describe the solution you'd like
However, you may want to have a background color in your diagram when working on a screen to reduce the strain on your eyes (some kind of dark mode), but you do not necessarily want this background color to be in the final diagram when you export it as an image.
At least, we should have the choice in the matter.
I would also like to have this possibility with the command line interface, for example a --no-background-color-export switch.

Describe alternatives you've considered
There are no alternative: if the diagram has a background color, it is also exported in the picture

Additional context
None