v0.1.0-alpha.64

Nacre's terminal background becomes white. It was a pale violet inside a
chrome ground that leans blue, and two different hues a few points apart
read as a mismatch rather than a relationship. The terminal is the
content, so on a tinted ground it is the white card.

The file explorer's disclosure chevron takes the chrome tint. It had been
AppKit's stock triangle, drawn in a system colour that ignores the ramp,
which on a themed panel is somewhere between invisible and wrong. The
history list already replaced it; both sidebars now disclose the same
way.