Let me be clear, I hate the whole Idea of the user selectible themes. I sometimes works opposite what we expect unless our site has only two theme options. One is light theme and another dark.
But, Talking about the automatic day and night themes are more like fancy lighting on a Lamborghini. Not all users love dark theme, like me. I get bored easliy reading with black backgrounds. If you are going to implement something like sunset themes, You should then provide an option to user to choose what theme to make default for him.