Day 20
A theme switch you steer with daylight. Drag the sun across the sky and every shadow on the page swings with it: long and warm at dusk, short at noon. Pull it below the horizon and the moon takes over.
Drag the sun around the sky, or use the arrow keys: 5 degrees per step, 30 with Shift. Home is noon, End is midnight. Below the horizon the theme turns dark.