Embedded UI: Dark mode style improvements

Improvements

• Enhanced visual consistency and usability in dark mode, including:
  • Fixed content overflow at card corners
  • Aligned card border styling with inner section borders
  • Updated ShareLinkPrompt wrapper styles:
    • Resolved corner overflow issues
    • Applied consistent header background
• Adjusted "Secured by Notabene" text color for better legibility
• Increased contrast on heading text for improved readability