Add high-contrast themes to general preferences

[ralfbrown]

No description provided.

[elstoc]

Would you mind looking up the appropriate darktable PR reference and providing a link to it when you raise dtdocs PRs please. Just helps with the admin.

[ralfbrown]

PR 17245.

[elstoc]

Thanks. If you could post them as links (darktable-org/darktable#17245) that would allow me to be really really lazy :)

[elstoc]

LGTM thanks

[elstoc]

@ralfbrown: One more thing to note here. I know you have the power to amend labels in darktable PRs but I'd prefer to do that myself when I'm satisfied the documentation is complete. Also generally I only remove the "documentation" labels entirely if I don't think a PR needs documentation updates. For completed documentation I remove the "documentation-pending" label and replace it with a "documentation-complete" label.