Use the page palette for default selection backgrounds instead of a
hardcoded blue color. Tint opaque selection backgrounds before painting.
Selected images stay visible through the overlay.
Use SelectionText for default selected text and HighlightText. Authored
::selection colors still take precedence. On macOS, use a light
translucent color matching other engines.
Update selection and system-color baselines for the themed colors. Add
macOS-specific screenshot baselines for tests whose expectations depend
on system colors.
Seed the page background fallback from the platform color scheme before
WebContent has painted. Update the same fallback when the platform theme
changes, and when the preferred color scheme is forced through the debug
menu, so unpainted web view areas use the matching Canvas color.
