I see three possible positions you can take regarding theonomy.

1. Theonomy is false. God doesn’t care about justice. Governments
can do whatever they want.
2. Theonomy is false. God doesn’t care about justice. Governments
can do what they want as long as they follow the moral law.
3. Theonomy is true. God cares about justice.

If theonomy is false, then it’s not just that God doesn’t care
about justice. It’s that there’s no longer a standard for justice.
We can no longer say whether a law is just or not.

To say the Old Testament civil law is unjust presupposes an absolute standard of justice. To claim the civil law is unjust is a self-refuting, self-contradictory sentence. There is no other standard for justice.