The automatic support means should a window request a dark version part of a skin and the author has not defined one (which will be the case with all the older skins), it will create a suitable one from the light mode versions.

So as an example if you had Windows in light mode and opened notepad on Win11 and set that to use the dark mode theme on itself then the notepad window will have a darkened version of the theme rather than the normal theme.

Authors can use SKS11 to define their own images and colours to use in this situation if they wish.

Each of the sections should have a button for you to define dark mode images on the image tab in skinstudio.

You do not define a new substyle, it is part of each substyle itself.

It does require you to be using Skinstudio 11.