Add a Theme selector in Display Options that cycles three presets with a live
preview: Custom (CPS-editable, default), BASIC 80s (Commodore 64 light-blue on
blue) and Terminal (green-on-black night / black-on-green day). Presets fill all
theme items for a cohesive look.
The selection is stored in the codeplug custom-data area (new THEME_PRESET
block), which is migration-safe (missing block -> Custom) and needs no settings
reset. CPS theme edits only ever change the Custom theme. Documented in MANUAL.md
and the README.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
8c9828e386