Just like anything in our software, we cannot provide a solution for every individual or edge-case scenario one will encounter. We can only do what's right for our customers, in this case, not apply an unnecessary cookie until a user accepts that.
As a theme author, it is your responsibility to work with your client to get the theme in an environment that you can review the issue. Whether that's duplicating the environment to a test, exporting the theme, setting up a window to test, etc.. etc...