[gnacho]

The theme switch was removed in v0.70.0 (issue #53), so nothing reads
general.color_scheme: it was parsed, validated and written back, with no
effect on the UI.

Change

• Drop the field, its default and its validation from GeneralConfig.
• Replace the old fallback test with one that proves the compatibility path:
  a config carrying "color_scheme": "banana" still loads, and the key is
  gone the next time the config is written. No schema_version bump is
  needed, the crate never sets deny_unknown_fields.

Verification

cargo fmt --check, cargo clippy --all-targets -- -D warnings and
cargo test clean, 739 tests passing.

Closes #246