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.