One key theme could be justice. In a civil action, the pursuit of justice through the legal system is often central. Another might be power dynamics, like how big corporations may have more power than individual plaintiffs. Also, the theme of perseverance can be important as characters fight long legal battles.
Typically, there are lawyers. They are the ones driving the legal action. There are also plaintiffs, the ones who bring the case forward. And defendants, who are being sued. Maybe there are also judges who preside over the cases.
For a civil action novel to be great, it needs a clear and important theme, like justice or social change. Strong writing that brings the legal aspects to life and keeps the tension high throughout is also essential.