If you need to make Dark Mode UI, you can put it into a new folder called "bg_dark" and then replace the code in the script.rpy or whatever file you added start to with the code here:
init:
# Set the default background to the light mode background image
$ renpy.defaultbg = "your_light_mode_background_image_filename.png"
label EnableDarkMode():
# Set the background to the dark mode background image
$ renpy.defaultbg = "bg_dark/your_dark_mode_background_image_filename.png"
label DisableDarkMode():
# Set the background to the light mode background image
$ renpy.defaultbg = "your_light_mode_background_image_filename.png"