Add Solarized Dark builtin theme

[HermesSantos]

Ship the theme as a bundled gallery preset.

[gitgusilva]

Thanks for this — the palette is solid, and it's good to see graph1..8 and graphMarker filled in, which is the part that usually gets missed.

This isn't the right home for it, though. Themes in builtins.ts are compiled into the binary: they need a GitBox release to ship, they carry no preview image, and users can't delete them.

The gitbox-themes repository is exactly for this. A theme is just a folder:

themes/<id>/
  theme.json      # the palette, plus meta.author, meta.description, meta.version
  preview@2x.png

The app discovers it through the GitHub contents API — no index file to update, no release to wait for. It shows up in Settings › Appearance with your preview and your name as the author, and users install the ones they want.

GitBox itself is moving the same way: the bundled presets are being dropped so the app ships GitBox Dark only as the offline default, with the whole catalogue in the registry.

So I'm closing this here — please do open it against gitbox-themes, the theme is welcome there. Happy to help if anything about the format is unclear.