Sooooooo apparently I just tried doing it on Samsung Keyboard and Dark Mode overrules the Light theme selection, switching to GBoard keyboard, go to keyboard settings, theme, and you should be good to go.
Use "Keys Cafe" in the Good Lock app from Samsung. You can style and fine tune the Samsung keyboard to your liking. Even just have look like "light mode" all the time. It has a light and dqrk setting that can be adjusted. If you have never used it, mess with it and try different things. You might like what you create better than what developers make.
Go to your phone settings, scroll down to general management, scroll to Samsung keyboard settings, then down to themes. There are a few basic themes in dark or light mode there.
