In a dark appearance with Pages Match UI Theme off, the locked section's notice
drew the dark theme's light text on the light page, and the welcome and empty
notebook notices sat on the chrome's surfaces, which a translucent backdrop
lets the desktop show through. They now fill with the page's colour and build
their text and buttons in the theme matching it: the light theme's on white
pages.

Fixes #67.

Assisted-by: claude-opus-5.5