Feature request: theme-aware code block highlighting for blog renderer

[gitcoder89431]

While drafting the first go_tui blog post, I noticed the code block chrome can follow the active site theme through CSS tokens, but the syntax highlighting itself is fixed by the selected Shiki/rehype-pretty-code theme.

Current behavior:

• Code block frame/background/borders can use theme tokens such as --surface2 and --line.
• Syntax token colors stay fixed, e.g. one Shiki theme like vitesse-dark, even when the user switches the blog theme.
• This makes code blocks feel slightly disconnected from theme variants.

Requested behavior:

• Support theme-aware code highlighting so each site theme can map to an appropriate Shiki theme or token color set.
• Keep the code block chrome token-based.
• Prefer a low-maintenance mapping, for example:
  • hackerman -> green/terminal-friendly dark syntax
  • gold-rush -> warmer dark syntax
  • void -> purple-friendly dark syntax
  • other themes -> sensible fallback

Why:

• The blog uses theme tokens everywhere else, so fixed syntax colors stand out.
• Personal/dev posts with lots of code should feel visually consistent across themes.
• This came up while QAing the Go TUI series article and comparing against the reference renderer.

Possible implementation notes:

• Investigate whether rehype-pretty-code can emit CSS-variable driven colors or multiple theme outputs.
• If not, select a Shiki theme based on current site theme at render/build time where possible.
• Add a fallback for unknown themes so existing posts remain stable.