- branding.tsx: `color="yellow"` → `t.color.warn` so light-mode users get the
burnt-orange warn instead of unreadable bright yellow on white bg.
- theme.ts: replace HERMES_TUI_LIGHT regex with `detectLightMode(env)` that also
sniffs `COLORFGBG` (XFCE Terminal, rxvt, Terminal.app, iTerm2). Bg slot 7 or
15 → LIGHT_THEME. Explicit HERMES_TUI_LIGHT (on *or* off) still wins.
- tests: cover empty env, explicit on/off, COLORFGBG positions, and off-override.
Splits the existing palette into DARK_THEME (current yellow-heavy
default) and LIGHT_THEME (darker browns + proper contrast on white).
DEFAULT_THEME aliases DARK_THEME, and flips to LIGHT_THEME when
HERMES_TUI_LIGHT=1 is set at launch.
Skin system (fromSkin) still layers on top of whichever preset is
active, so users can keep customizing on top of either palette.
Refs #11300.
