I notice that SO has a dark mode but it doesn't seem available on Web(whatever-we-are-going-to-call-it).
Or am I missing something?
This site unfortunately does not have a dark theme, as far as I know.
A similar question about dark mode for Stack Exchange sites was brought up a couple months ago on Meta Stack Overflow. The original feature request for dark themes for Stack Exchange sites was posted a little over ten years ago.
As of March 2020, the company's official stance is as follows:
For now, we have no plans to bring dark mode to the many sites across the Stack Exchange network. Many of the designs on our sites have been around long enough that converting them to dark mode would require redoing the artwork completely. We would prefer to avoid giving anyone across our network a substandard experience and we don’t want to change those elements without the input of these communities.
Introducing Dark Mode (beta) for Stack Overflow - Stack Overflow Blog
Some of the responses in those threads have some unofficial user styles as workarounds, though.