bvbfan Changing the default style have the same effect. You have to remove the solid pattern for the background to have effect, which revert the actual background to dark (if you have dark theme that is).
Think of the work sheet as a three layered paper, in the bottom you have the paper/base color (white), on top of that you have the background color (whatever you set it to, or "transparent") and on top of that you have the foreground color (whatever pattern and color you chose, or none if you don't want to use any pattern). In Libreoffice you can change the paper color, but as default it's white no matter what theme you use.
An idea is to add an option in the setting to choose what base color to use, as you can do with the grid outline. The idea is that you can change the base color to reflect the paper color in your printer (usually white, but sometimes you might wanna use yellow or red paper?) You can't set the background color to fx. red to reflect he color of the paper, because that will print out red on the paper to. But you might still want to see how it will look like before you print it on a red paper (text color and stuff like that).