Fix search modal visibility and backdrop issues (#1291)

- Fix keyboard shortcuts visibility in light mode by updating text
colors
- Change CSS muted color from dark-text-muted to light-text-muted for
light mode
- Update Footer component with proper light/dark mode color classes
- Fix dark mode backdrop opacity from solid to semi-transparent (40%
opacity)
- Improve overall search modal UX in both light and dark modes


Amp-Thread-ID:
https://ampcode.com/threads/T-08b8bac6-c754-47b3-9c3e-82e8580318f9

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Co-authored-by: Amp <amp@ampcode.com>

Author: enriquegh