Mirror new CSS tokens from light.css into dark.css

[Copilot]

The parent PR (#342) introduced 17 new CSS custom properties to light.css with no corresponding dark-theme overrides, causing dark mode to silently fall back to light values — producing low-contrast or visually inconsistent UI.

Changes

src/styles/themes/dark.css

• Borders: --color-border-accent: #9999dd (lightened from light's #8888cc for dark-background contrast)
• Spacing/Sizing: Added 7 dimension tokens (--spacing-xxxs, --spacing-xxs, --spacing-btn, --border-width-xthin, --border-width-sm, --border-radius-xs, --border-radius-sm) — values match light theme (layout/size, not color)
• Shadows: Added 6 shadow tokens with elevated opacity for dark-mode legibility:

  --box-shadow-active: 0 1px 2px rgba(124, 77, 255, 0.3);   /* light: 0.2 */
  --box-shadow-focus:  0 0 0 5px rgba(124, 77, 255, 0.35);  /* light: 0.25 */
  --box-shadow-modal:  0 2px 10px rgba(0, 0, 0, 0.5);       /* light: 0.3 */
  /* …overlay, hover, popup similarly adjusted */

• Overlay: --overlay-bg-muted: rgba(0, 0, 0, 0.5) (vs. 0.3 in light)
• Misc: --opacity-input-disabled: 0.75, --grid-min-column: 30em (theme-neutral, kept identical)

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