Remove data-theme attribute when mode="auto" instead of setting it to
"auto". Pico's OS-dark selector is :root:not([data-theme]), so any
value (even "auto") prevents dark-mode Pico styles. Add Pico form
element background vars and color-scheme: light to match active
theme.
Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>