Introduce dark theme

[kelson42]

This issue is a follow-up of openzim/overview#53

ZIM should AFAP provide a "dark theme" alternative to the standard "light theme" using the CSS color-scheme and prefers-color-scheme primitives.

This new CSS theme should come either from the upstream (if possible) or should be invented (always if possible).

[dhyaniritesh89]

Hi, I’m interested in working on this.
Which CSS file would be the best place to start for adding dark mode support?

[Asif-rehman012]

Hi, I’d like to work on this issue.

I’ve read the earlier discussion and understand that the agreed approach is
to support dark mode either via upstream CSS (where available) or by
inversion when necessary.

I plan to start by identifying the relevant CSS files and testing how the
current light theme is applied, then propose a minimal, incremental change
before opening a PR.

Please let me know if this approach sounds good.