Neat. If it were that easy to do on a production website we would have done it already.
If there's already two themes in SMF, why not upload another one?
As I mentioned, we could upload a dark mode theme, but it would be a generic one that doesn't have any of the road-specific imagery that the Button Copy theme has. More to the point, though, it sounds like when SMF 2.1 comes out we'll have to redo the themes then
anyway, so it would probably be best to just handle it then.
And, I want the dark theme to be when the sunrises and sets.
I want you to paint your bike fuchsia.