Convert site theme to monochrome grayscale palette

[Copilot]

The site theme still used a multi-color Dracula palette, while the requirement is fully black-and-white rendering. This PR replaces the palette with grayscale-only values while preserving existing layout and style semantics.

• Theme palette update

  • Replaced color variables in src/templates/style.css with grayscale equivalents (#000000–#ffffff range).
  • Removed all chromatic accents from links, headings, metadata, and code surfaces by remapping their existing variables.

• Visual hierarchy preserved in B/W

  • Kept the existing semantic variable model (--d-*) and selectors unchanged.
  • Assigned distinct gray levels across semantic tokens to retain contrast and readability (instead of collapsing everything to white/black).

• Scope

  • Single-file, CSS-only change in src/templates/style.css.
  • No template or runtime logic changes.

:root {
  --bg-color: #000000;
  --fg-color: #ffffff;
  --d-comment: #c7c7c7;
  --d-cyan: #9a9a9a;
  --d-green: #bdbdbd;
  --d-orange: #8a8a8a;
  --d-pink: #d9d9d9;
  --d-purple: #ababab;
  --d-red: #767676;
  --d-yellow: #f0f0f0;
}