Customizable colour palette for the vector UI toolkit (packed 0x00RRGGBB).
ui_theme(...) sets it; every widget falls back to these and accepts a
trailing r,g,b override.
Current hue phase (radians) for line strokes, or None when the cycle is off.
Elapsed is computed in f64 (epoch seconds are huge) before the caller casts to
f32, so sin keeps precision across a long session.