At the moment, with current AI capabilities, it would be very hard for someone who doesn’t know how to program to come up with a complete working Discourse plugin/theme of reasonable complexity. And I hope we never implied that something like this would be possible, as it would be a frustrating experience.
That said, for someone with a junior-level programming knowledge, using either a common LLM, a code-specific one, or something like GitHub CoPilot can make the journey definitely easier, handling a lot of the boilerplate for you. Combining it with an existing plugin/theme and starting with small changes sounds like a good idea for someone willing to learn.