xterm's default ANSI 16 is tuned for dark and reads candy-bright on the
light glass surface (vivid cyans/greens). Ship the canonical Solarized
palette (Schoonover) for both modes — same 16 accents either way, only
fg/cursor swap between `base00/01` (light) and `base0/1` (dark), so a
prompt's colors look uniform across a Shift+X toggle.
Background stays transparent in both modes — Solarized's cream/slate
backgrounds would fight the glass.