v0.6.3: 0.6.3: support Config UI X dark mode in the Custom UI

Reported via GitHub issue (VERSA 15 + ETHM-1 Plus): when Config UI X
runs in dark theme, almost everything in the plugin's Settings page is
unreadable — text and inputs all use my hardcoded #24292e on top of the
parent iframe's dark background. The reporter could only read anything
by highlighting it. The bug hits every dark-mode user; VERSA only made
it visible because that's what they were testing on.

Refactor the CSS to CSS variables defaulting to the existing light
palette, and add a `@media (prefers-color-scheme: dark)` block that
swaps them to GitHub-style dark tokens. Touched: body bg/fg, cards
(default + .on), banners (default + .warn), inputs / selects (incl.
disabled state), badges, hover states, sticky save bar, focus ring
opacity.

Light mode pixel-identical to 0.6.2.

https://claude.ai/code/session_01WeievjpLmBhNMj36Sxcbdb