I do agree with Dinesh V Divekar. However, it should be taken into account that the organization's culture is the totality of norms, beliefs, and attitudes of any organization's workforce.
As an HR practitioner, it is your responsibility to inform your senior management that they are the focal point of ethics in the working environment. Nevertheless, in most management books, it is written that people are the main factor in shaping organizational behavior.
But practically, most of the time, it is senior management who shape the organization's culture. For example, if any employee acts against the organizational rules and regulations, and the policy prohibits that action, although in this situation, senior management may use their authority and power to overlook it. Here, the main factor in shaping and building the organization's culture will be the senior management. There is a term like "physical contract," which the employee signs on the date of joining with the organization, and it has a legal aspect. Another one is the "psychological contract," which is an unwritten document where both parties have some expectations from each other, such as honesty and integrity.
My strong recommendation to you is to first talk with your senior management, and then you can draft any type of policies, i.e., dress code policy, communication policy, code of conduct, etc.