Add darkmode support for default theme

[tom21100227]

This pull request introduces dark mode support for two CSS files: github.css (for code highlighting) and default.css (for the Techfolio theme). Also updated bootstrap.css to the latest version.

Dark mode support for syntax highlighting (github.css):

• Added a dark mode section to .highlight, changing its background color to black and adjusting text and background colors for various syntax elements (e.g., .gd, .ge, .gi, .kc, etc.) to ensure readability.

Dark mode support for Techfolio theme (default.css):

• Introduced dark mode variables for base colors, elevated surface colors, links, icons, and page sections, ensuring consistent theming across the site.
• Updated styles for specific components like body, .navbar, footer.navbar, .text-muted, .btn-outline-dark, .card, and .card-footer to align with the dark mode design.