The off-state thumb reused the palette's primary foreground, which resolves
to near-black in dark mode; against the translucent track it read as
disabled, while a disabled control differs only by half opacity. Give the
off state its own thumb color through the new `--dsw-alias-switch-thumb`
alias: white in light mode and neutral-bluish 400 in dark mode.
The plugin management info button also becomes an 11px caption-colored icon
in a 20px square, keeping the 4px gap beside the subtitle.
Refs #5347
Yif
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