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.