Firefox supports following the system choice, between "light mode" (default) and "dark mode". Chrome is still lagging behind in this regard, here a user AFAIK always be presented with the "light mode" regardless of the system choice.
In Firefox and presumably other browsers supporting this, the demo looks like so:

The easiest solution would probably be to just disable "dark mode" in the demo altogether, instead of "pretending" to support it.
Firefox supports following the system choice, between "light mode" (default) and "dark mode". Chrome is still lagging behind in this regard, here a user AFAIK always be presented with the "light mode" regardless of the system choice.
In Firefox and presumably other browsers supporting this, the demo looks like so:

The easiest solution would probably be to just disable "dark mode" in the demo altogether, instead of "pretending" to support it.