This is most likely part of this: Is OJS GDPR compliant?
As it is mentioned there, I am planning to make a general GDPR plugin that adds, among other things, the functionality you described. I am planning to do this without editing any core files, so it should work with any theme. But at the moment I am waiting for PKPs response and see what their results are and what they are maybe going to do with OJS to meet the requirements.