how can I change the background color of an adobe illustrator document?

Answer 1

Go to File menu > Document Setup. In the Document Setup window click on the drop down arrow on the left and click on Transparency, then select Simulate Colored Paper.
Click on the top grid color swatch and you can choose a new color within the color picker.
This will change the Artboard color and will add a transparent tone to the art too, as if printing on colored paper.