default json colors are unreadable in ted

[tig]

My background is dark in my terminal. ted foo.json is basically unreadable.

Read CLAUDE.md and ./specs

[tig]

Tracked by #132. The plan addresses this directly — TG's Dark theme will populate VisualRole.Code* entries via gui-cs/Terminal.Gui#5311, so xshd-supplied colors (e.g. JSON's Bool, Null, Number) get themed against a dark background. Full plan in specs/syntax-theme/spec.md.