No apk file will work. Note 9 users dont have dark mode version of galaxy app store. Even if you try the apk externally it will force you to update and it will be white again. Have got reply after writing to Samsung who have forwarded the request to their developers. So it's a catch 22 situation. why should anyone go to developer mode. Though it may force dark mode but will invert colours too for sites. Example Amazon web page looks crazy and it is not a solution.