Civil (or civilian) law is the legal relationship between private persons. These individuals can also be companies, but in any case the state is not involved. Since the fundamental rights apply only to the state, at first sight there is no room for a constitutional complaint.
However, one must take into account that the decision of civil disputes happens through state courts, which of course have to respect the fundamental rights. This conflict is solved by the doctrine of the so-called “indirect third-party effect of basic rights”. Although the fundamental rights are not within the private law relationship, but as a framework legal order provide a foundation for the court’s decision. In the case of non-defined legal terms, which make a balancing of interests necessary, fundamental rights guarantees can be used.
If the constitutional examination is not correct, the decision may be unconstitutional. Then it would be repealed in the context of the constitutional complaint procedure.